When "posting" fish, make sure they are in a water-proof envelope.
They cannot exceed a certain size as you then get a large increase in the postal charges.
To add insult to injury, they must be packed in crush proof envelopes, that will not be squashed as they go thru the letter counter.
As for keeping them warm, dunno, they don't make heat-paks that small, so guess you'll have to use priority mail to speed up delivery. :roll:
Other than that BG, I think the answers prior to mine, will answer your question.
Alan
ps. don't even think of sending my guppies as I suggested