Pay Invoice with than Paypal?
Seems like Paypal is the only option for people to pay. May small businesses already have existing merchant service providers other than Paypal. Can you include way so a business can setup a custom payment gateway URL with template items that can be transferred to the gateway?
Here, Microsoft Invoicing will replace "ms_" is replaced with actual values.
To handle paid invoice notification, Microsoft Invoicing need not take return URLs (see API request)