You can automate your client payments using merchant services so that clients can pay and recharge their accounts in real time. You can setup the invoices to autocharge their credit cards when bill comes due as well. Please note not all merchant providers may support periodic payments.
We can setup standard PayPal (Custom development) for additional charge.
Here is the list of merchant accounts that will work with Porta Switch (always check with Support as the list can change due to software upgrades and such) :-
4. Chase Paymentech
13. Payflow Pro
19. WorldPay Invisible
20. WorldPay Junior