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.
Good luck and let us know what works