SAT Subject test registration concern

I have registered for one SAT Subject test but on test day I took two, when am I supposed to pay for the additional test’s fees and how much will it be?

With College Board’s SAT tests, you pay a fee per test date. You can take up to three subject tests based on the one fee you have already paid, i.e., you don’t owe anything extra.

Are you sure? When I was registering it added $20 fee whenever I added another subject test.

Your correct I am wrong. I was misreading some info from CB. What CB says is that it will bill you for the extra test, see https://collegereadiness.collegeboard.org/sat-subject-tests/taking-the-test/test-day#changing-tests So I would assume you just wait until you receive a bill, although I would check your test registration on line and see if there is anythwere in it that shows a charge added that can be paid online.