I received an invoice for something I didn't buy. How can I tell that it is genuine? It says to click a link to get a refund and that link then takes you to a page that asks for credit card details. Do I ignore ?

Yes, a scam. Do not react in anyway !!

If this came by email: delete the email, then delete it from the email trash box.

If this came about via an other way:

Quit Safari, restart Safari while holding the Shift key, in Safari menu do "Clear History".

then, to be sure,

download AdwareMedic( now MalwareBytes AntiMalware): http://www.adwaremedic.com/index.php

and "scan".

Lex

my pleasure 🙂