I went to the post office yesterday to speak with the supervisor. The woman at the counter I spoke to said both the supervisor and manager (the latter of whom I've spoken with previously) were out, but gave me a number to call today. She also gave me advice: the next time, after putting in a Vacation Hold card, call the post office a few days before the trip between 7:30-9 AM and ask to speak directly with my mail carrier. Let her know that I've dropped off a card and make sure that she got it. (Carriers arrive at 7:30 and head out on the road at 9.)
I called the supervisor and manager today, but neither were in. However, the woman who answered was helpful, understanding and solicitous. Whether that helps, we'll see. She was also very honest. (Hence my leaving out her name, to protect the innocent.) She said that she was well aware of this problem. The problem, she explained, is that when you drop off the Vacation Hold card downstairs, it doesn't always get upstairs where it's needed. "It's like downstairs and upstairs are two different worlds," she said. (I could only think that finally America had a potential PBS series for "Masterpiece Classic.")
Her advice was to use the online Vacation Hold service, because people seemed to have better luck with it. And she agreed with the advice I'd gotten about calling my mail carrier before leaving. (She relieved my concern that the carrier would consider this noodging. "I have customers do this all the time," she said. "And we'd rather know that someone is out of town, rather than us wonder what's going on.") She also suggested calling after leaving town, as well, to double-check that the Hold went into place. "Some people do this, too, and I then after I speak with them, I go around and make sure the carriers know."
Clearly, this is no way to run a pop stand, but it was good to get the advice and -- get the confirmation that no one would get their nose bent out of joint by noodging. It's ridiculous, extra work that shouldn't be needed, but assuming it works (and that's still an assumption, after 20 years...) then it's s worth it compared to the uncertainty of having an overflowed mailbox while on vacation.