This computer is here because PayPal doesn't provide one, so if you want to charge your customers for the PayPal fee you have to do the math yourself.
I've spent a whole day figuring out the formula (ok, I didn't pay
attention in the classroom) and even had to go consulting the
excellent WolframAlpha engine.
Thanks to dapal in #gfoss and the people in #math for reminding/explaining me that (y - ay) == (1-a)y.
I hope this computer here can be helpful to others as a workaround to the PayPal disservice. If you want to look at the math just view this page source code, it's actually very easy.