My guess as to the late-ness is because either Tom got drunk and forgot to send it, or the staff of the local postal agency got drunk and they forgot to do their jobs. I'm thinking the former as it would be highly unlikely that the entire staff would get drunk.
Am I right? Am I right?!

...I want a BYOND Membership! =(

Don't forget that they have to ride back!

[Edit:] Though I guess they could catch a plane home... But what to do with the bikes? Plus, it sounds like Tom's gambled his travel money away! Maybe they can sell the bikes for money for plane tickets?

After looking around a bit... I don't think that Tom didn't send the postcard late because he didn't want us to know that the adventure was ending soon, because after all, I've read through the first few posts on Dantom del Byondo's blog and all I've seen is that Mike H. came up with the idea for the contest since everyone thought it was so that must mean that Tom doesn't even know about it. I think that the postcard was sent to Sioux Falls, SD first for some reason like no nearby airport or something to that effect, but I doubt it was because there was no nearby airport, and then sent to Mike H. hence the stamp on the back of the post card. Also like some people think that it was because of the typo in the address, that wouldn't make sense because even Mike H. said that the address wasn't even close to correct so that wouldn't matter.

*edited because of a typo in my second sentence that basically changed the meaning of the whole thing*

Ends on day 31 with 1686 mileage.

Day 36
1735 miles.

Day 31, Last Day, 1720 miles.