What if you use the form to write all of your products and quantities to a sheet with email and unique ID or row id. In essence a homemade shopping cart or a build order process. Then show the list to the user, have them add to the actual cart, and then have them complete their purchase. I’m envisioning that the unique ID or row id could be your SKU (Product ID). That way you have all the information you need to link the app:sales sheet to the order sheet.