The cost of mailing a first-class letter in the U.S. went up to 39 cents yesterday, an increase of two cents. This was part of an overall USPS rate increase. Perhaps you already knew about this, but I had no idea a rate hike was coming until I stumbled across a reference to it at Lifehacker today.
It’s been in the news, but more obviously, it’s been posted at Post Offices for a month or more. When I went to the local post office to mail your birthday gift (which unfortunately you received late,) I purchased a book of the new stamps so I would be ready on January 8 and not unwittingly mail something which would arrive with postage due. But be warned: there is likely to be another hike next year, and I imagine it will round off to 40 cents. That figure makes more sense anyway.