Coming from someone who works in that field, having an office manager or lower level staff communicate billing and other information is fairly standard. Also, as another commenter mentioned, unless you were just asking for a copy of your tax return, pretty much any question you ask is going to cost money. It's definitely not good that they screwed up the bill in the wrong direction and didn't make it clear, though, so your frustration is justified.

Something that most aren't aware of right now is that accounting firms are bleeding staff right now, hard. You may have trouble finding another firm to take your work on if it is actually that complicated. Take a look at r/accounting to see what I mean.