The rates are going up for US Postage retail rate letters effective May 14. 2007.
But if you are sending out cartoon batches, like I do, and paying for a couple of extra ounces -- the rates are actually going down.
A letter -- regular first class mail -- increases from 39 cents to 41 cents beginning May 14th. The per ounce rate, however, drops from 24 cents per ounce to 17 cents.
So, a 3 ounce package that costs me $0.87 today, will only cost $0.75 in May!
Rates and fees tables here. USPS decision posted on their Web site.