Sci-Fi. Trainee/ new hire clerk at intergalactic post office: problems with various races culminate with an alien customer committing suicide. Read around 1980 or earlier. Spoiler.

[Niels]

Trainee/new hire clerk at intergalactic post office: problems with various races culminate with an alien customer committing suicide. Whole day turns out to be a training simulation. Read a couple of times, perhaps 45 years ago or more.