I ran a small FBA business for a year nowhere near any scale and it was an interesting lesson. People who weigh in on these tech topics should actually try these things out to understand them better.

It's nothing like listing on eBay where there's some haircut off the top and that's that.

There are fees for - accepting inventory, holding inventory, returning unsold inventory, shipping sales, processing returns, destroying returns, transaction fees, ads to promote your listings in market, and probably 5 other things I forget. Depending on the fee it is - fixed, % of $, weight based, volume based, or some combination.

The fees change constantly with not much notice. So every time you think you've got just the right size/weight/price balance you get screwed. And where else are you going to go?

Like Uber drivers, I imagine some % of FBA sellers don't know they are losing money in real time. You need to do some decent accounting to track as all these different fees hit at different times. It's not like Amazon gives you the data & tools to track your all-in costs per sale.