Single car shipping, the goal should be to keep the weight below 13 ounces,
above that there is a big increase to Priority Mail, and about $2.00 more. 1st
class is also non-zone rated, which means a cross crountry shipping will not
cost extra over a near destination. Priority Mail charges hefty extra fees to
farther away destinations.
I will frequently wrap a HO kit with kraft paper. Most freight cars will then
ship 1st class. I sent two kraft wrapped flat cars to a STMFC member; to keep
the total under 13 ounces, I had to remove one of the instructions. The same two
cars in a box would have been about $7.00 instead of the $3.80.