KTH's official address is just KTH, 100 44 Stockholm. But since the '10' prefix already implies Stockholm [1], you could feasibly shorten it to just '100 44'.

Actually, it looks like Chalmers is the same way - the address in the footer of their website is just "Chalmers University of Technology, 412 96 Gothenburg" (with the '41' prefix already implying Gothenburg).

[1] https://www.postnord.se/vara-verktyg/sok-postnummer-och-adre...

Not that I send many paper letters any more, but long ago I took to simply writing my return address as my (unique) last name and zip code which has worked a few times (not that I’d know of any failure cases).

For US post office boxes you can just write the full 9-digit zip code.