Back in the midst of their worst times, I sent them a check by certified mail. I kept an eye on the account and they never showed receipt of payment and my check didn’t clear. When I called them a month later (to give them plenty of time), their response was that just because I had a certified receipt didn’t mean I had actually put a check in the envelope. Funny how that check was deposited a few days later.
I did recently pay my taxes using the online method, and everything shows as being paid. I definitely kept the receipt I got when I made the payment, just in case.