An important lesson from Jim F's experience: the more contact you have with a human, the more likely someone can say "no." I almost always machine-print (meter) my mailing labels and postage, and I'll bet they only examine a random sample of mail.

When I worked for government, we had a game going: see how many letters we could mail to ourselves with bizarre postage. The key was to type the address. We started with using the last pre-rate hike postage. It went through. Then we tried a 10-cent stamp. It went through. also worked: 1 cent stamp. Canadian stamp. Irish stamp. Easter seal. The extra tab in a stamp booklet that says "this is not postage." A hand-drawn stamp. These were mailed over the period of a year, and were deposited in letter boxes (not handed over at the counter).