I just won an auction for a postcard and the seller is in Australia. When I check the cost of postage on http://ircalc.usps.gov/ It shows two prices that confuse me...

Airmail Letter Post
4 - 7 Days $0.80 Max. length 24", Max. length, height, depth combined 36"

Economy (Surface) Letter Post
(Rate for pieces 16 ounces or less)
4 - 6 Weeks $4.05 Max. length 24", Max. length, height, depth combined 36"

Does anyone know why the second choice is more expensive than the first one?
Is there a reason why the first choice is inappropriate to mail an envelope containing a postcard?

I must be missing something. The prices make no sense to me.

Thanks for the help