The service is different though. If you use Amazon as your warehouse, on the lower tiers, you just ship inventory and they sort it by SKU .. for all stores! So some supplier could store a bunch of counterfeits with the same SKU, someone orders from your store, and your customer get a counterfeit (even if you've only shipped real products). .. This is one big reason most sex toy manufacturers won't sell to retailers that have Amazon stores.
I'm going to guess UPS keeps warehousing separate per vendor.
I'm going to guess UPS keeps warehousing separate per vendor.