I use a mat with a back and plastic envelope. It is then ready for them to buy a frame. It is very professional and usable in stands as well.
A kit of 25 single mats + manilla backings + clear polybags of 18x24 inches (with 12½ x 18½ opening) is 115 $US, i.e. $5/piece, at Matshop.com (or .ca if you are in Canada).
Sometimes, I also used only the backing + envelope.
Yvan