Quick update, I found the tax payment myself, it had been applied to the wrong tax year (I didn't enter it incorrectly, the receipt shows the correct tax year). I still haven't heard back from them via email and have tried several time simply leaving the line open on the customer service number for hours on end but they have never picked up. Whether it was them or the IRS I don't know, but judging from the BBB comments it seems more likely it was them.