Well if I’m left with no option but to go to the counter at the post office it’s precisely because I have no idea how to do what I want with online resources.

E.g. post a flat cardboard package containing art that I made myself to a foreign country. The envelope sizes they sell at USPS stores don’t match the rates they post on their website. Often, the clerk can help a little bit with filling the customs form. The website says that you need a license to export art to foreign countries, the clerk doesn’t.

Stop blaming other people, start blaming the system: we need more clerks, better payed and better trained clerks.