It is an rule that you can not charge extra for paypal. 2.9% + $0.30 for up to $3,000 = could be added to shipping, but you would get into hot H2O with if you only added it to paypal buyers.
Paypal does save the .39 stamp for the seller. I will bid a bit more on something that I can use paypal on. Saves the stamp and gives you a little safety from scams.