“Is there a way to let customers choose from a variety of different templates AFTER they have selected a product and clicked the Customize Order button? Think Homepage > Business Cards > Full Color Business Card (Product Page) > Customize Order > view and choose a template to edit at this point.
You can create an eDocBuilder template and, using the Image Picker feature, let them choose from a variety of backgrounds. The merge field layout and locations would be unaffected and unchanged. If you are using the Interactive Designer, you could also let them move the text fields around so they can adjust for the different designs.
Related article: Ch. 008. Template Assets - Graphic Files and Image Picker Groups