i wrote the address directly on the banana with a sharpie, taped up the whole banana with packing tape in case of squishing, and stuck some stamps on it.  for those who stuck a stamp on it and are surprised it hasn't been received, it costs about $2.00 in stamps depending on the weight of your banana.  so maybe you didn't use enough postage.

i bought a greenish banana because it was not as squishy and i didn't mail it very far, so when my friend got it (2 or 3 days later), it was actually still green and barely bruised.