On these items Amazon fully realizes they can charge 90% and the seller is still making a profit, so they do.

If you're selling appliances or TVs or computers the % is not nearly that bad.

The warehouse and shipping fees on such large & heavy items would be onerous would be my guess.

Yeah, I've done enough FBA to rank somewhere between a serious hobbyist and an actual lifestyle business.

There is a sweet spot. If you're selling lightweight crap items, even the small shipping and fulfillment costs still tend to dominate. If you sell huge stuff, the shipping will bite, but not as bad as you'd expect -- Amazon has very good rates. The big problem for bulky items tends to be warehouse fees, and also the logistics of getting it to the FBA warehouse in the first place.

I'd say 1-3 cubic feet is exactly where you want to be.

Here's the actual fulfillment fees, if you're curious (notice how reasonable the rates are in the "Large Standard" range):

https://sellercentral.amazon.com/help/hub/reference/external...