It really is to protect the sellers because they must pay any paypal fee if the buyer pays with paypal.
So if you sell 1 card for .18 cents and you charge .62 cents to ship in PWE. .80-.44 stamp = .36 -.35 paypal fee = 0.01 You make 1 cent on the transaction... Even if you make 5-10 cents is it really worth the time?