I put mine in a regular plastic fish bag. 1/4 or 1/3 water, basically enough for the shrimp to be covered if the bag is lying on its side. The some java moss or stem plant for them to hang onto. And a drop of prime. I get my boxes from work, because USPS flat rate boxes are more expensive. Make sure no side of the box is over 12". I ship in summer and just line the box with what ever I have on hand. Newspaper, packing peanut, those air bags. A cheap trick if there is lots of extra space in the box, is the blow up ziplock sandwich bags, then tape the seam. Ship priority.