If you read the article that the OP linked to you'll see that the Post Office has the largest fleet of vehicles of any single company.

The question is that if the Post Office were really interested in going green, why don't they deliver more mail by bike instead of just putting a picture of a bike on a stamp?

The mail at our house is delivered on foot, - sort of. They park at one end of the block, deliver mail on both sides, then drive on to the next block. Better than when I was growing up in the suburbs when they never left their cars. Still, they could at least pick more efficient vehicles. The trucks they use in our area are basically Chevy Suburbans.