Mini prints and postcards, how to ship them?

Hi sellers,

I want to send a few mini art prints (A5) and postcards to FBA. What do you think is the best way to do it?

I've put them inside a cello bag with a cardboard back. Is that enough, or should I put them inside a brown (manilla) envelope for extra protection? or would that be overkill? I'm selling them in sets of 9, so I don't mind adding this extra cost per set.

Thank you!