The 12x9x6 box is the most popular size for shipping, but the 15x11x7 is also handy.
Keep in mind that the smaller the box, the less expensive it will be to ship. You don't want to get all 15x11x7 boxes just because you have a couple of larger animals, as it will make the rest of your 600-800 gram shipments more expensive.
You don't have to use SYR supplies, but your supplies do have to meet our Shipping Standards. That includes a like new box with at least 3/4" insulation and temperature considerations for heat pack and cold pack use.
A shipping kit is a complete package for a one-off shipment. If you want to build up a stock of supplies, that is not the way to go : )
You can order in larger quantities, and order each item separately- insulated boxes, bags/delis, heat packs, cold packs, etc.
Before SYR we did build our own boxes using insulation from Home Depot. We used the thick white insulation board, 1.5" thick, and cut it to fit our boxes. Works great, but it is a bunch of extra work and time. The 1.5" insulation is great for cold winter shipping though, for sure. You can also double up the 3/4" insulation for better cold resistance if you like during winter shipping (or summer for that matter).
Feel free to contact us directly for packaging questions as well, always happy to help!