This is not strictly to do with OSC, but you are risking getting on the wrong side of Paypal, the law or both:
The price paid by a customer has to cover all your costs, unless you explicitly apply other charges or fees and many payment systems including Paypal will not allow you to do that to cover their charges. If a customer complains to paypal after they've paid, they'll get the charge refunded from your paypal account so they are quite likely to do this, especially if it means they are then getting a really good deal. Your account will then be suspended.
If you add hidden charges you will end up dealing with Paypal, card companies, Trading Standards (or whatever your equivalent is), etc. as it is usually illegal to advertise one price and charge another.
Remember that your customer is actually the one paying the paypal fee via your price, so Paypal (etc) is not costing YOU anything at all, its just reducing your profit margin. Therefore the only way you can "lose any money" through Paypal or another payment system is if your prices are too low to cover the cost.
beside all that, most of your customers will be happier with an honest pricing system with no hidden charges and no nasty surprises at checkout.
:)