A 'flat' or 'flats' is common collecting vernacular for any item that is...well, flat. Stickers, stationary, paper, postcards, tags, magazine pages, anything that can be easily stored or scanned! I don't often go out of my way for flats, but I am partial to clear files in particular.
Clearfiles
Clearfiles are folders for holding papers. They come in several different paper sizes, for your convenience. I like collecting clearfiles since it's often a cheaper way of obtaining items with art on them. Often, art featured on clearfiles is also on more expensive products like TCG accessories. It's an easy way to have functional and pretty art.