The account credits functionality can be further extended by adding an option to top up the credit using voucher codes.
Admin generates unique voucher codes with certain amount and expiration date assigned to each voucher. Codes can then be printed and distributed for selling.
Client buys a card, and can then use that voucher code and enter it in the 'Add Funds' page, the corresponding amount will then be added to his account credits balance.
This is useful in regions where there's no access to global payment gateways (.ie: PayPal) or no credit cards accessible there.