Salvinia is so soft, it is unlikely to be damaged much in transit. And I'm sure it will survive for a few days at least in very humid conditions, so I think I'd wrap them in very damp paper and then in plastic. Just be sure they have enough space so they won't be crushed, and a sturdy package that can't be crushed in either.
Frogbit, otoh, is rather brittle. The larger the leaves the more likely they are to be damaged, so I'd want to ship younger, smaller plants. A puffed up bag of air, similar to the way fish are packaged to take home from a store, might be a better arrangement for something like frogbit.
I had a lot of frogbit packed in small bags with about an inch of water for an auction, and not all of it sold. I was too tired to unpack it when I brought the rest home. The bags were in a spot where they got plenty of light, but stayedl in the bags for about 3 more days. Many of them were starting to look a bit sad when I did unpack them. All the runners were snapped, many leaves had broken edges, and a fair number of the oldest leaves were starting to turn yellow. Once I trimmed off the damaged bits, the rest was ok, and is now growing well again back in the tank. I suspect it would not last as long as some other plants in the total darkness of a package. Mind, I haven't ever shipped any, so I can't say for sure, but I'd want it delivered pretty quickly to be on the safe side.