If you don't have a credit card or if you prefer not to use credit cards as a payment method, it's never a problem with the WunderCounter. When you're on the billing page, scroll to the bottom and you'll find the "Cheque or Money Order" option.
To get to the billing page, choose "Billing/Payments", which is under the "Options" menu. You can choose the same billing periods as you normally would when paying by cheque/money order. All payments can be sent to the PO Box.
If you have sent a payment by mail, feel free to let me know via email that the payment has been sent so that I can confirm with you when it arrives.