In addition to paying Heap's or Torch's monthly bill using a credit card or pre-paying with PayPal you can now pre-pay using Google Checkout.
In Heap under the "Settings" tab select "Account":
In Torch under the "Settings" tab select "Account":
Or if your account has recently expired, you can reactivate it (https://www.wbpsystems.com/reactivate):
These links will pop open a window where you can select the amount you want to pre-pay: