In the end I was able to send back the unused generic Advair (which was 250mcg/25mcg, which apparently is the same as 230/21) and order Advair itself (well, actually Seretide, as it's coming from the UK).
In the meantime I had to go back to the doctor here in Portland and get a new prescription for Advair, and pay another $390 for an inhaler to tide me over. Sigh. The good news is that I'm not having adverse reaction to the 'proper' Advair, and it's definitely helping my wheezing.
It is a palaver, however.