Order Fulfillment vs Purchasing API: Which to Use (2026)
ShipBob ships inventory you own. Zinc buys from retailers. Learn which API fits marketplace fulfillment vs purchasing.
Fulfillment API search results assume you already own the inventory. ShipBob, Amazon MCF, and 3PL docs pick, pack, and ship stock in your warehouse. They do not buy a product from Amazon or Walmart as a shopper.
A purchasing API is the other direction. Your software sold something (or an internal user requested it) and now needs to purchase the item from a retailer you do not operate. Zinc is that API. It is not MCF.
- Two APIs that share the word order
- What an order fulfillment API does
- What a purchasing API does
- Side-by-side comparison
- How to pick in one question
- When you need both
- FAQ
If you already know you are on the buy-side, the companion how-to is automate orders across ecommerce sites.
Two APIs that share the word order
"Order" in ecommerce software means two opposite objects.
Sales order: A shopper bought from you on Amazon, eBay, Shopify, TikTok Shop, or your site. You must ship it. The API talks to a warehouse, a 3PL, or Amazon MCF.
Purchase order (retail): Your app must acquire the product from a retailer website. The API talks to amazon.com, walmart.com, target.com, or Best Buy as a buyer.
Marketplace developer docs almost always mean the first one. Amazon's MCF integrations guide is about connecting sales channels to Amazon's fulfillment network. Blue30's fulfillment API guide is about Shopify orders flowing into a warehouse, then to Royal Mail or DPD. ShipBob's developer orders guide starts with POST /order so ShipBob can pick Light Roast Coffee you already stored.
A purchasing API starts with a retailer product URL, not a warehouse SKU.
Mixing them wastes a quarter. Teams buy ShipBob because the search said "fulfillment API," then still have a person on Amazon Business for tail SKUs. Or they buy a purchasing API and expect it to print UPS labels for inventory in their own warehouse.
What an order fulfillment API does
An order fulfillment API receives a sales order and tells a warehouse what to pick. Inventory already exists in that warehouse. You inbounded it, or a 3PL received it on your PO from a manufacturer.
Typical capabilities, from the vendors in this space:
- Import orders from Shopify, Amazon, eBay, Walmart, TikTok Shop
- Allocate stock, split shipments across fulfillment centers
- Create labels or hand the carton to a carrier
- Push tracking back to the sales channel
- Receive returns into the same warehouse
Amazon Multi-Channel Fulfillment (MCF). Amazon's supply chain integrations guide describes MCF as a 3PL: pick, pack, ship off-Amazon orders from inventory you stored with Amazon. Connectors include a free Shopify app, ISVs, and OMS tools such as Rithum, ShipStation, Listing Mirror, Sellercloud, and Pipe17. Amazon also publishes MCF APIs for custom builds. Amazon's MCF developer page states an average click-to-door of 1.9 days (NielsenIQ, cited by Amazon). That speed applies to units already in Amazon's network.
ShipBob. ShipBob's apps and API page lists turnkey imports from Shopify, BigCommerce, WooCommerce, Amazon, Walmart, eBay, and more. The orders integration guide has you POST a DTC order with your reference_id, recipient, and products that already exist in ShipBob. Tracking comes back via order.shipped webhooks or polling HasTracking=true. Cancel works until the shipment is picked. This is warehouse execution.
Blue30. Blue30 describes the UK 3PL version of the same loop: store order, warehouse, courier, tracking. They name Shopify, Amazon UK, WooCommerce, eBay, TikTok Shop, Magento as sales channels, and Royal Mail, DPD, Evri, DHL as carriers.
Sellercloud and EasyChannel. Sellercloud is an OMS: listings and operations across channels. EasyChannel exposes GET /v1/orders and order.created webhooks so you can send sales into a WMS. Both assume you will fulfill those orders from stock you control.
Apideck. Apideck's 2026 ecommerce API roundup is a unified merchant API: Shopify, SP-API, WooCommerce, Walmart Marketplace, Etsy. It normalizes orders you received as a seller. It does not check out at those stores as a shopper.
Fulfillment APIs are the right answer when the tracker question is: "I sold something on eBay or Amazon and need to ship my stock."
What a purchasing API does
A purchasing API places a shopper checkout at a retailer you do not own. You send a product URL, a ship-to address, and a maximum price. The provider pays the retailer and returns confirmation plus tracking.
Zinc is that API. It is not a 3PL. You do not send pallets to Zinc. Zinc does not allocate your SKU from a warehouse. It buys the public listing.
Use it when:
- A procurement tool needs to buy from Amazon, Staples, or Best Buy after approval (procurement automation API)
- A gifting or rewards product needs to ship a physical item from retail (corporate gifting API)
- An AI shopping agent must complete checkout, not open a tab
- A store sources a SKU from US retail rather than a wholesaler feed (scale dropshipping without hiring)
Official Amazon, Walmart, and Target APIs do not do this job. They are seller, marketplace, or affiliate surfaces. See Amazon API and Walmart API.
Zinc fields that matter in this comparison:
- Create Order with
products[].url,shipping_address,max_price(cents),idempotency_key is_giftto hide packing-slip prices (fails withgift_option_unavailableif the retailer has no gift checkout)- Webhooks
order.placed,order.tracking_received,order.delivered - Returns API for orders Zinc placed (not for ShipBob inventory)
- Test mode before live retailer checkout
Zinc is not MCF. If the unit is sitting in an Amazon fulfillment center under your seller account, call MCF. If the unit is sitting on a Walmart shelf in a listing you do not own, call Zinc.
Fulfillment API vs purchasing API
| Fulfillment API (3PL / MCF) | Purchasing API (Zinc) | |
|---|---|---|
| Inventory | You already own it, inbounded to a warehouse | Retailer owns it until checkout |
| Trigger | Sales order from a channel you sell on | Purchase request from your app, agent, or ops tool |
| Example request | ShipBob POST /order with your SKU LIGHT-ROAST | Zinc POST /orders with https://www.amazon.com/dp/... |
| Who picks the unit | 3PL or Amazon staff | Retailer's fulfillment (Amazon, Walmart, Target, etc.) |
| Tracking | 3PL/carrier events, then you upload to the sales channel | order.tracking_received on the purchase Zinc placed |
| Returns | Into your warehouse (Loop, Narvar, ShipBob returns) | Retailer return via Zinc returns API |
| Packing slip | Often your brand, or MCF unbranded | Retailer's slip; is_gift hides prices or the order fails |
| Amazon MCF | Yes, this category | No |
| Good for | "I sold it on eBay, ship my stock" | "My app needs to buy it from a retailer" |
| Vendors | ShipBob, Amazon MCF, Blue30, Sellercloud | Zinc |
Apideck sits beside the fulfillment column as the unified store API. Useful if you must read Shopify orders and Walmart Marketplace orders through one schema. Still sell-side.
How to pick in one question
Ask: Does the unit already sit in a warehouse I control (including a 3PL or MCF)?
- Yes. Use a fulfillment API. ShipBob if you want a 3PL network. Amazon MCF if the stock is already in Amazon's network and you want off-Amazon orders shipped from there. Sellercloud or EasyChannel if the missing piece is channel aggregation, not the warehouse itself.
- No, I need to buy it from a retailer. Use a purchasing API. Zinc.
Secondary checks:
- Are you the Amazon seller of record for this unit? If yes, and it is inbounded, MCF. If no, and you would otherwise click Buy Now, Zinc.
- Do you need your brand on the box? Fulfillment APIs can do custom packaging. Retail checkout will look like a retail order.
is_giftis not custom packaging. - Is this tail spend or a hero SKU? Hero SKUs belong in a 3PL. One-off SKUs belong on a purchasing API so you do not inbound dead inventory.
Amazon's MCF guide cites Capital One Shopping research that 62% of shoppers consider shipping speed critical, and Baymard that 21% abandon because delivery estimates are too slow. Those numbers argue for a fast 3PL when you hold stock. They are not a reason to skip holding stock and skip purchasing automation. They are two different investments.
When you need both
Many real stacks use both APIs.
Example: you sell 50 core SKUs from ShipBob. When a customer orders an accessory you do not stock, your OMS creates a Zinc purchase to the customer's address (only if that channel's TOS allows retailer sourcing). Tracking from Zinc is written onto the original sales order.
Example: HR approved a $120 monitor. Coupa has the requisition. Zinc buys it from Best Buy and ships to the office. No 3PL involved.
Example: you sell on Amazon with FBA for in-stock items (fulfillment), and a separate internal tool buys competitor-retail samples for QA (purchasing). Do not run the samples through MCF. They were never your FBA inventory.
Do not pretend Zinc is MCF. MCF fees, inbound appointments, and inventory reports do not exist on a purchasing API. Purchasing max_price and retailer availability do not exist on ShipBob.
FAQ
What is an automated order fulfillment API for online marketplaces?
It is software that takes a sales order from Amazon, eBay, Shopify, or similar and tells a warehouse or 3PL to ship inventory you already own. ShipBob, Amazon MCF, and Blue30 are in that category. Sellercloud and EasyChannel feed those warehouses by aggregating channel orders.
I sold something on eBay / Amazon and need to ship my stock. Which API?
ShipBob or another 3PL if the stock is in their warehouse. Amazon MCF if the stock is already in Amazon's network and the sale happened off-Amazon (or you are using MCF's supported channels). Not Zinc.
My app needs to buy the product from a retailer. Which API?
A purchasing API. Zinc Create Order with a retailer URL, address, and max_price. Not ShipBob, not MCF.
Can Zinc replace Amazon MCF?
No. MCF fulfills units you sent to Amazon. Zinc buys retail listings. If you do not inbound inventory, MCF has nothing to pick. If you do inbound inventory, Zinc is the wrong tool for those units.
Can I use Apideck or Shopify APIs to buy from Amazon?
No. Apideck and Shopify Admin APIs operate on stores you control or seller accounts you authorized. They do not complete shopper checkout at Amazon or Target. See best ecommerce purchasing APIs.
Final recommendation
If you hold the unit, fulfill it. If a retailer holds the unit, buy it.
Install the Universal Checkout skill and paste this when the item is not in your warehouse.
Read the buy-side how-to next: automate orders across ecommerce sites. Docs: Create Order, integrations, pricing.
