My local Sam's Club used to (they still may?) stock multi-packs of 1lb bags, that are stored in a what appears to be (a very thin) mylar packaging. The use by date is roughly 2 years from when they were purchased, and I have opened them well beyond that and they were fine. I do not recall any absorbers, though nor do I have any on hand to read off any possible preservative ingredients, I'm sorry.
They came in resealable packages and we used them well past "the day" (mostly baking but were still fine by the handful as far as I am concerned).
Concord grapes grow well in the region, I'm hoping my vines produce well this year for some home-made raisins and to add some more flavor depth to our wine.