This has made me curious because of a thread. It is not about that thread.
I have an Etsy shop. Over the years I have juggled things about to maximise sales.
When I have charged, say, £30 + postage I sell fewer items than when I sell, say, £35 with free postage. (Or more accurately, with postage included.)
YANBU - You don't get free postage. You either have a product with postage factored into the price or you have a product with postage cost applied separately.
YABU - you do get free postage, sellers make less profit because they absorb postage fees into their costs
I exclude massive massive companies from this btw. Eg Amazon - they are so huge they they could make a penny per sale and still be quids in.