I DON'T know!
Receipts fade to near nothingness in my purse, which is kept in my bumbag and is intermittently warm and maybe slightly humid.
I would suggest cool and dry might be best. If I could be arsed, I would try experimenting with one batch in a clean, dry bag in the freezer and another in the fridge.
My fax (yes, I still have one!) has a thermal printer and I could easily print a few sheets and keep in varying conditions.
BUT I'M FAR TOO LAZY!