Hmm, I could swear it just did it when you checked the debug box at admin/store/settings/quotes/edit, but trying it out it doesn't seem to show up. I don't know, it's late. Anyway, stick the following line in uc_usps.module in the function uc_usps_quote(), just after the first call to uc_usps_rate_request():

That should show your request.