When talking about e2e tests, it matters where the e's are. If they are at the public interface of "the service", then, indeed, you'd test against "credits".
But ideally, there'd be e2e tests that test the consumers' experience, where the 'e's are the web interface, emails, CLI and other things consumers click on, read, enter etc. This is where a "test currency" makes sense.
You can have your billing service billing in the test currency for e2e (and probably with a value that does not match the USD) so you can test "ok this should cost 1 credit so in the end I should have 18 $currency"
When talking about e2e tests, it matters where the e's are. If they are at the public interface of "the service", then, indeed, you'd test against "credits".
But ideally, there'd be e2e tests that test the consumers' experience, where the 'e's are the web interface, emails, CLI and other things consumers click on, read, enter etc. This is where a "test currency" makes sense.