I'm not brave enough to post on other boards, but wanted to seem helpful.
Please take a sample to the post office before you buy postage and maybe even see if you can get it in writing that the cost will not change.
I was told the forever stamps would be fine. One oz is what it can be. Mine turned out to be 1.4 and I had to buy some 20 cent George Washington stamps to add to it. Also, response cards have to be at least 3 1/2 inchs high and 5 inchs long. And you should be able to use the postcard cost rate but might want to have that confirmed before you purchase.
Good Luck!