Hi Jim,
There are a bunch of custom features in the ecommerce package. Basically, if a feature is unimplemented, the code does not trigger any related activity.
Creating a callback might be a practice that works for other locations.
For ecommerce, you might find it more consistent to just add the code and have it not trigger when the feature is unimplemented --consistent with existing code strategy.
Each product_id is an acs object_id, so a product_id / certificate map table should be all that is needed to create triggering code.
If you want to integrate the code further, you could create a product_category_id for the certificates and identify the category_id via a parameter or something.. again, where the code doesn't trigger activity if the feature is not implemented for a particular installation.
Anyway, I'm confident you (will) have something that works well.
cheers,
Ben