russbrett you are on the right track. The smaller box will hold one deck and the larger will hold 2. You will need a small piece of bubble rap or a bubble bag to snug the decks up. I know that KWP uses these type boxes exclusively, and for good reason. Knock on wood and cross my fingers. I have never received a damaged deck that was shipped in one of these. Just tape these boxes up and apply the postage. They work great.
As far as the bubble bag- go with a slightly bigger one like the
http://www.uline.com/Product/Detail/S-680/Bubble-Bags/4-x-5-1-2-Self-Seal-Bubble-Bags or the
http://www.uline.com/Product/Detail/S-10549/Bubble-Bags/4-x-5-Bubble-Bags-Open-End. You will want to shoot yourself, or somebody else about the third deck you try to stuff in the 3x5 bag.
You may want to also pick up some bubble mailers. I wouldn't go any smaller than these.
http://www.uline.com/Product/Detail/S-9985/Bubble-Mailers/Uline-Self-Seal-Bubble-Mailers-0-6-x-10. You can put the smaller box in one of these for international shipping. gives you plenty of roof to attach the custom's form.
You may also want to pic up a bigger box for international shipping- like a
http://www.uline.com/Product/Detail/S-433/Indestructo-Mailers/9-x-4-x-3-Indestructo-Mailers or similar. Something that will hold at least a brick plus padding.
Keep in mind ULine does charge shipping to ship your shipping supplies. I recently purchased about $300 worth of supplies, and my shipping was around $60. That's about 20% of the purchase price.
When shipping domestic, anything over 2 decks. 9 out of 10 times it's cheaper to ship USPS Priority Mail-- and not just flat rate. the regional box A1-
https://store.USPS.com/store/browse/uspsProductDetailMultiSkuDropDown.jsp?categoryNavIds=shipping-supplies%3ashipping-boxes&categoryNav=false&navAction=push&navCount=0&productId=P_RRB_A1&categoryId=shipping-boxes can cost as little as a small flat rate box to ship, but typically $7-$10. The best thing about priority mail boxes is that they are free. You don't even need to go to the post office. Just order them on-line, and they will drop them on you front step for nothing-$0- zilch.
In the end shipping is probably the biggest underestimated cost of a KS project. It's nice to get that big old check from KS- just keep in mind it will take 20%+ of that check for shipping/shipping supplies. Also don't discount your time. If you work a full time job- realistically you will get 30-40 packages done per night. So if you have 500+ backers?
Not sure who you are using as a printer, but for example- if you print with LPCC- they will charge you to ship the decks to your US address- then you pay shipping again to you backers, and your time. If you use their fulfillment. they take care of everything(at a cost) and you get charged to ship the left over decks to you. If there is a down side to using their fulfillment- I recently received 2 decks in a bubble mailer from them- no damage luckily. I don't know if they have different levels of packaging or not.
Hopefully this helps you some-Good luck on your project