It's not mail art yet but all it needs is an address and a cancelled stamp, so I posted it just for the inspiration. Although I would leave the three-dimensional flowers off, as they pose a risk to getting the envelope caught in the machinery.
LOVE THIS paper bag pocket - so many uses. Put treats/gifts in the larger back pocket with a note card in the front; or as shown, gift card/cash in front with greeting card or treat in pocket, etc. I'll just staple sides and cover with paper.