Buying 'cash' with cash?

[Quillink]

This isn't so much an adoptables question as a php general question, but does anyone know how to make an automated buy-currency-with-cash system?
I have the dual-currency system and store running A-ok, but I'm clueless as to how I can make a means for users to buy 'cash', as I call it, with real money via Paypal.

See, Arvyre's got two currencies; "Clix", which you earn when someone clicks your pet, and can be used to buy general items and pets. And then there's "Cash"; Obtained by paying real money and used to buy premium items and pets.

At the moment a user can pay $X and I will manually add X00 cash points to their account, but I'd like to make this automated. So, does anyone know how, or can at least point me in some direction? :)