Charging PayPal Fees

I am a paypal user myself so don't take this as an attack. I note several Paypal users charging the fees for the use of the service. Does anyone else realise that this is an illegle pratice? You can add the price to cover your losses, however, you can not offer the product for two different prices. Not that I really feel this will lead to any arests, I just wanted to inform you all.