Currently, our kits are about the same size, so we have decided to keep shipping simple -- using a small Flat-Rate Priority Mail box, which uses a Free box, Free Tracking, and Free Insurance. We looked into regular First Class mail, but while it is a couple dollars cheaper, we then have to buy a box or padded envelope, add Tracking to the shipment, and insurance would add even more. All that work to now save you just pennies to ship you a package slower? We didn't think so, either.
Multiple items will require a larger, non-flat-rate box, but we only add $1 more per item (we will cover mailing costs if it is more than we ask for).