Given the timestamp of your post, that's almost certainly due to the fact that the quote was requested after the last pickup - so the Express Mail can't go out until tomorrow, and won't arrive until the day after that. Hence 2 days. The response from the USPS server shows the timestamp and takes the time (adjusted for the timezone of the mailing ZIP code even!) into account.

If you turn on the debug, you will see the result coming back from the USPS server says 2 days - there is no mix up on the Ubercart end.

BTW, you will need to upgrade to Beta 2 to do this because the debug info was messed up in Beta 1.