USPS is what I use. Medium is around $14 and large is around $18. The boxes are free, so it would be best to go to PO and pick up both sizes in both shapes (flat and square), to see which fits better. I sometimes use the space bags that you vacuum the air out of to make it fit a box. If you kneel/push on the bag, you can get it flatter, and shape it, if needed. It usually arrives in 2 to 3 days.