If you must bag them, use a paper bag and even then I'd give them a few days exposed to open air, circulating air.

I left my echinacea seed heads for the chickadees this year or I'd send you some, wouldn't be surprised if someone else has kept more than they need though...You may not have to buy.

I harvested buckets of Echinacea seeds but while they were drying inside the garage, some sort of tiny worm ate each one from the inside out and left me only the empty seed husks. Sorry I don't have any to send you. You might check the Seed Exchange and maybe someone will have them for trade or postage.

There are a few I don't let dry more than a couple of weeks at most after harvesting - those go into sterile moist vermiculite in the tiny ziplocks. But these are exceptions to seed storing and not the rule - include dicentra scandens, epimedium, hellebores, saruma, cyclamen, trillium, couple of others...

I keep my seeds in plain old drug store type envelopes, in a plastic shoe box size bin, in the basement (dry and cool). A few envelopes collected late this fall are still in my car's center compartment because they never made it inside, and are experiencing any freeze that happens. I worry more about heat than cold for raw seeds. I kept a batch in the shed by mistake, and the shed can get pretty hot in the summer time. I have little hope for those, but will still plant them.