What pack are you using. I think I saw that you have a ULA? So the Conduit, I assume? The Gossamer Gear 1/8" thinlight is what I'm going to be using as the back pad for my Gorilla. It's 60" long, so it seems it could also serve inside the pack using the burrito method. Or they have a 3/8" pad for 5-7oz.
At risk of going off topic, I don't think it's a big deal to use REI/MEC's return policy of you have the genuine intention of keeping the product, but then find out in the field it doesn't work as expected. A pad you should be able to figure out at home pretty well though. I work 2 blocks from an REI, so I spend many lunch half-hours wandering the aisles, and particularly in the shoe and backpack depts(2 things its hard to get a feel for without field testing IMO) I hear the staff reminding customers teetering between yes/no quite frequently about REI's return policy, so it's not like it's something they don't want you to know about/use. As I heard one guy tell someone regarding a backpack, you can use it and still return it, just please don't set it on fire or anything, (though he said they could still return it if they did).
The only jerks are the people who buy a tent to use at Burning Man with the intent of returning it after, and then can't even be bothered to even attempt not to cover the floor/fly/netting with burn marks, so REI can't even resell it for a decent price.