Southern Louisiana, where I live, also is quite humid,
Costa Rica probably more so, but when a coin develops full-blown
bronze disease (which, from your description, I can hardly doubt your coins have) in the space of days or weeks, I must wonder whether the coins hadn't been superficially cleaned, but not effectively treated, for pre-existing
bronze disease before they were photographed, sold, and shipped. I have
had to be careful of my bronze coins, of course, putting sachets of silica gel in all the storage boxes and not economizing on the air conditioning in the rooms where they are kept. With these precautions, only coins that arrived already with bad symptoms have been troublesome. I'll find the page with a coin of mine that
had bad BD, with photos of progressive treatment.
Pat L.
https://www.forumancientcoins.com/board/index.php?topic=43730.0Be sure to read all 8 replies. Don't try quick fixes. Also see other threads on
bronze disease, but, as I said, drastic, "sure-shot" fixes are likely to make matters worse.