Although you can add as many payment types as you like within WriteUpp,  you'll notice that 2 of the pre-configured options as marked as 'Protected':

These payment types cannot be edited or made inactive as they are used by WriteUpp in the onward recording and reporting of payments. 

Multiple Payors isn't an option that appears when you register a payment against an invoice, but it is used by WriteUpp to indicate that an invoice has been fully paid by multiple payors.  You can see an example of this in the Status column of Finance -> Invoices:

Stripe is selectable when registering a payment against an invoice:

And will appear in the status column of both the Invoices and Payments tab of the Finance page to indicate that a payment was made via Stripe:

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