(via PayableForm http://www.crazychro.me/gform)
Well, just create a normal text field, with the question title as “paypal invoice”. Like this:
This field won’t be displayed when your visitors filling the form. After the form is filled, PayableForm will generate a uniqe invoice id for every entry on the response spreadsheet. You can then use the invoice id to search on Paypal site. Like this: