What is the best kind of packaging to ensure safe shipping for greeting cards?

I think rigid mailers would be good! They may require extra postage though because of the rigidity.

I’d go with rigid mailers too. I sell greeting cards and for awhile I was just sending them in a larger A7 envelope with a piece of heavy card stock to protect it, but I had a card get damaged in the postal system. So far, the rigid mailers have the best track record and the postage cost isn’t that much more.