I need to know the default url that a form submission goes to that has a payment (without me having to put through a purchase). How do these urls start so I can setup a goal conversion for them? Is it /forms/ ?
Yes, the form will process to /forms/cases.ashx?form=<alias_name> with a {{this.formSubmissionData}} result just like non payment forms.
So you could render your tracking code based on conditions of the form response if needed (ie: Success or Error).