Hi! I use 6x8 rigid mailers from Amazon to mail my cards / stickers. I use first class mail package via Etsy labels. I use this because I prefer the tracking info it provides. I have gotten 3 or 4 surcharges added to my package and I don't get it. I know these envelopes are thin but I have even stuffed the top full with packing peanuts to try and bulk it up a bit. As I have seen where they say oh well it should be a letter and not a package... so basically it's EASIER for them so they charge me $4... Now recently with this new method I've gotten 2 new surcharges. Most of them have gone through fine, but I don't get what I am doing wrong here...