I have three cards I enclose in with my Etsy sales package plus a note of thanks. I made these little holders in the same paper as my matchbook note holders that I also include. (K&Company Life's Journey)
They are 3.5 x 4.5 inches and I make two scores in the middle at 2-1/8 and 2-3/8 inches. I use a corner rounder on all four corners. The pocket is a 1.5 inch square. It takes about 14" of ribbon to make the handles. I close it with a Tim Holtz mini clip :)
There are a lot of debates in the Esty Virtual Lab meetings about whether to send a thank you, receipt or other marketing materials. I love receiving extras but do not think a thing about it if I only get my purchase. These are fast, easy and just the right size for the recipient to carry ten or so business cards in it. I do hope they are reused.