One is a marketplace programme, one is a 3PL
Walmart Fulfillment Services is Walmart's in-house fulfillment programme, in the same shape as FBA. ShipMonk is an independent third-party logistics provider serving DTC and multi-channel brands. Comparing them on fee tables alone misses the point: WFS buys you placement on Walmart, and ShipMonk buys you flexibility everywhere else.
What WFS actually gives you
Badging and placement. A WFS-fulfilled listing generally converts better on Walmart than a seller-fulfilled one, and Walmart handles delivery promise and returns intake. If Walmart is a serious channel for you rather than a side experiment, that advantage is difficult to replicate with an outside 3PL.
What ShipMonk actually gives you
Channel breadth and control. A 3PL fulfils your Shopify store, Amazon FBM, eBay, TikTok Shop and Walmart seller-fulfilled orders from one place, with your packaging and your inserts. For brands where Walmart is one channel among several, that consolidation is worth more than badging on one of them.
The inventory-splitting problem
Run both and you are holding stock in two places, forecasting twice, and discovering you are out on one channel while overstocked on the other. This is the practical cost that rarely appears in a comparison table, and it grows as you add channels.
The structure that avoids it
Hold one pool upstream, prepped and ready, and feed WFS with what Walmart will sell while shipping every other channel directly from the same stock. You keep the Walmart badge where it pays and avoid duplicating inventory everywhere else. That is the arrangement we run from our owned Austin facility.