Okay, Just got my poly bags yesterday. 3mil, I am glad with the purchase.
However, to confirm with what others said. Yes, you do need weight on the board to get the posters flat. So I will have another hardwood board on top of the foamboard and some additional weight on top to flatten out the posters first. Then back to vertical storage for space reasons.
I am putting the posters in the poly bags, and back into the foamboard and clip storage method for now.