Visa Extension possible in Manila Airport
When I flew into the Manila International Airport just a month ago, I went to the tiny immigration office right before you clear immigration and paid somewhere around $60-$70 to get my visa extension stamped into my passport.

This is WAY easier than going to the offices in town and cheaper than using a travel agent.

FYI, if you're going to extend it do it when you land, it took me like 8 minutes!!