if during your next international trip you're in a city where us embassy/consulate is located, take 1/2 day off, walk in, ask for extra pages, and you're done. no charge and usually done while you wait or few hours at most. Some locations will try to tell you 3 days and if you have the time, then no big deal, but if you don't, just state your case and they will make an exception.

did it twice, in tokyo (told 3 days), done in 2 hours, and in warsaw, done in about 1/2 hour.

my extra pages were taped in the middle of the passport. they stamp it with the amended date.