How do I detect a store checkout page with a chrome extension?

Question

I am developing a chrome extension where I would like to show a popup to my users whenever they visit an e-commerce store's checkout page. I understand one option is to get a list of checkout page URLs for all the merchants I support, then detect that page with my extension but it would be a pain to gather thousands of URLs.

Is there some easy method for detecting a store's checkout page?

Also, as a bonus question, do you have any ideas for how I could detect a store's confirmation page? Without gathering a list of URLs or regex matches.