How to Ship a Maximum Card Safely
When I ship maximum cards, sometimes they are irregular sizes for which I don't have a form-fitting plastic sleeve. Other times, the cancel is sticky enough, or the card slick enough, that I am afraid of smudging the postmark. In those cases, I make glassine sleeves like this. They're a bit labor-intensive, but it's a great method.
Take your maximum card and two #6 glassine envelopes
Cut off the left side of one envelope and the right side of the other with scissors
Cut off the triangular flaps from each envelope with scissors
Slide the maximum card inside one envelope so that its upper-left is in a sealed corner
Flip the other envelope — horizontally and vertically — and slide it over the card and first envelope so that its lower-right is in a sealed corner
Center the card inside the envelopes and fold the triangular flaps in half
Use the folded flaps to cover the exposed gaps between the envelopes and tape over the tips of each flap to seal and protect the card. At no point should the adhesive tape touch your card or be near an exposed gap.
Tape your packaged maximum card, top and bottom, to your invoice (or a blank sheet of paper)