Why do you want this?
I can’t see any good reason. It’s much better for both usability and SEO to have the titles in the permalinks.
But if you REALLY want this, you can use “/shop/%post_id%.html” as a product permalink base. You change this under “Settings” -> “Permalinks” and then at the bottom under the section “Product permalink base”. Put the following as a “Custom Base”:
/shop/%post_id%.html