Thanks for the response Joyce!!
In terms of weight they'd all be fine as a large envelope since they're just under 4 ounces (I have a postal scale). I'm thinking that what would bump it up to a package rate would be the thickness. Its just a bit weird since clearly when people mailed them to me some postal workers chose to send em as different rates?!?
Also wondering... if people put their return address instead of mine and there's not enough postage does that mean it will just go to them and they'll have postage due??