If you have 3 or more cars, weigh them all together and calculate shipping to see what it is without the USPS boxes. Simple as that. I've found more often than not, using your own mailer is cheaper. Using the USPS boxes is great when the price actually saves you, which is about the $5 mark.
I typically just go with 9x6 bubble mailers. You can fit up to 4 cars and ship for around $3, so very affordable. This is usually what you'd do if you were sending your entries in to a race. This is what I'm sending my 2 Red Ball cars today, actually.
Now, if you're a host and then need to ship all the entries to the next host, that's when the USPS boxes come in handy. The medium size box can ship quite a few cars if you pack them in there and price to ship will be $12 (or $5 if you get creative with packing).
Bottom line is, you pay per weight, not per size. Use a bubble mailer or an old cereal box (a la Smitty) and they'll both work just the same and cost the same.
One last tip for mailing unopened cars...cut the cardboard down to just the blister part and send that. Unless you're mailing to a collector, the cardboard don't matter to them and it'll just get tossed anyway. Trim cards down as much as possible to save on the size of box/mailer you need.
good tips, thank you
I've found, since I'm usually buying cars online anyway (ebay and elsewhere), just hold onto the boxes! I usually have the size I need that way.
And, I usually wrap the cars in paper towels and put them in ziplock bags.
my 2 cents :)