Goo Gone can be bought at the hardware store.
My two cents worth ~ leave used on the envelope, scissor cut leaving a quarter inch border and use peelable hinges.
As far as unused self adhesive stamps ~ buy the sheet, mount with corner mounts (like the ones used on photos) on nonacid card stock and insert into a nonacid sheet protector. For WAG, again buy the sheet and buy Harco Sheet Files to keep it in. For single WAG and s/a use mounts but store in a moisture free area.
