Paypal

Okay, I've been out of the selling game for a number of years, so maybe I'm just not informed. However, I thought that paypal only charges 3% if a credit or debit card is used? If checking is used there are no transaction fees I thought. So why does EVERYONE on this site and Audio Asylum ask for extra money when using Paypal? This seems rather disingenuous to me. If you want to cover paypal charges then make if for when Paypal actually charges you, if you want to make a few extra bucks then raise your price. It just irks me. What am I missing? Are there charges I don't know about?