It means "Fulfillment By Amazon" - you sell products in web store and Amazon ships them.
But I'm afraid that "fixed price" is nothing desirable to use as a name. What is the advantage of "fixed pricing"?...Maybe I don't get it...
I think fixed price products sell quicker. I read somewhere setting BIN price for a domain generates more sale. In fact, all products on Amazon are BIN only. We can't negotiate on their price. So, may be fixed price is good!