From Shopp Documentation

This template provides the layout for the product page. The product page is a detailed view of a single product and usually includes nearly every detail of the product such as the full description, product image gallery, product details and specs, and pricing. The product.php template mostly uses Product Tags.

Product-specific Templates

You can create distinct templates for specific products by making a copy of the generic product.php template to use as a starting point. Simply make a copy of the product.php template and re-name the copy to a formatted name that includes the product ID number:

product-id.php

Replace id with the product ID number.