Once clicking on Submit - can each person be given a reference number? Refernce number must be included in the spreadsheet. Each person needs to make payment and we need them to use their reference number so that we can allocate the payments received correctly.
Each Google Form submission has a timestamp. You could use it as the reference number. Another value that you could use is the link to edit the response. For further details see Show URL used to edit responses from a Google Form in a Google Spreadsheet by using a script