The manufacturer that I'm currently working inventory up on has 2 price tiers. We have the best price tier. No one buys their goods cheaper than we buy them.

Product X has a cost of $13.46.
Seller Dumbass is selling it for $15.99 FBA Prime.

15% seller Fee on $15.99 is $2.40, leaving 13 cents above cost to pay for shipping, and make a profit.

Even in a manilla envelope, first class, said product would cost $3.84 at Commercial Plus Pricing through USPS, even on Prime.