Amazon's third party seller fees aren't trivial even for their FBM service. Amazon labels don't incur this cost just like B&M retailers don't have to pay slotting fees to to get their products on their own shelves. However other retailers charge this fee because shelf space is a scarce resource. Amazon doesn't lose this revenue when adding their items to the product listing.

Amazon also has 50% of e-commerce marketshare and nearly double Walmart's share of all retail sales.