Thanks all, again very much appreciated. Yeah, ironically it turns out we did provide them with the best we could, they were happy until the end, and euthanizing the second bird (not an easy decision...) was the most "humane" thing to do, as the bird must have been in terrible pain by then.
Thanks for the further info on cleaning. I had been reading up yesterday, and got confused as you whether you can or can not disinfect the area they had been living in.
I know there's a lot of posts in this thread: we did contact the shop immediately after the birds had died, and after a first knee-jerk reaction of one employee ("there's not a single bird here that got sick too"), they made sounds like they wanted to make good, and mumbled about unscrupulous breeders. After the diagnosis, this all makes sense.
To make matters more interesting (well, not the best choice of word), a friend of ours who lives near the shop, buys the food for her pet rabbit there, and is planning to buy a bird too, noticed last week that all birds that used to be in the shop were gone, replaced by new ones...
Although technically it's my wife's money, I am starting to feel like they indeed owe me/her a full refund, as they sold us two young birds that had 0% chance of survival, and a contaminated cage and toys etc.
Furthermore, given how contagious and persistent this virus is, I'm now wondering how many birds in that shop caught it, and how many owners who looked at other birds may have infected their own healthy birds. That's a lot of heartbreak...
I will inform them today, although I'm still wondering if this could have happened without the knowledge of at least the breeder. Can that happen? I'm having a hard time to believe that, and the literature seems to indicate that often it's breeders who don't want to let go of their "investment" who promote this virus.
I don't want to be unreasonable, but I really think a full refund is in order, and I think it's fair to not want other birds from them.
Something that may interest you: they did not find a secondary infection. The birds tested negative for all known bacteria, parasites, and other viruses. So they really died directly from PBFD causing malnutrition and anemia.
I also wonder if there's an official body where we can send our information/confirmation/autopsy report, as I'm sure this is potentially valuable information when it comes to disease control.
Finally, we did look at shelters, but that was before we knew about PBFD. My head is spinning a bit, but I'll keep that option open.
Also: no "new" birds unless I have a certificate and a thorough check by our vet. (and a thorough decontamination...)
To be continued...