Originally Posted by Paul-Xensia

Unless Protx will store and perform the recurring payments for you (a la Worldpay FuturePay), you'll need a billing package that can store the card details (personally I think this is very bad) and do it that way.

(I've not looked at Modernbill functionality so I don't know).

With ClientExec you can take two approaches, either use an auto-payment module that does the above (only some of their modules support this) or just let the clients manually pay each invoice.

I've just finished writing a new module for CE and I've opted for the latter.

Also remember you won't be able to use CVV with stored card details as CVV data is not supposed to be stored long term.