From the description I read in the App Store, Cardhop is not a plug-in, at least not like the ones you are describing in Mail. Cardhop may piggyback off of Apple's Contacts, but it seems to be a standalone app.
On a Mac since 1984. Currently: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.
