Hi,
we are currently trying something similar - our solution was to use contact lens cases (not my idea - I saw Christophe Leterrier (@christlet ) showing a picture on twitter). The cases are big enough for the coverslip to fit, but too low for it to flip over
I added a bit of NaN3 to the PBS, and in addition sealed them in.
We also tried to ship coverslips as small PBS-filled foil pouches -that also worked OK-ish.
Our EM people often remove mowiol from coverslips after confocal imaging for CLEM before embedding and EM prep by simply immersing the slides in buffer or water - that might also work, but I am not sure how good the staining would be afterwards.
Cool to see the lens cases trick being useful! We’ve used it for 12 mm and 18 mm round coverslips, sometimes between US and France, without problems. I recently presented this during the EuroBioImaging weekly webinar: