For some reason, no one has ever compiled an online list of post offices in Metro Manila. And by post office, I mean a place to send mail and not just the government-owned PhilPost. So I'll be writing about the different places where you could send postcards in NCR.
BTW, sending postcards locally will cost about half the price of sending a postcard abroad. The price depends on your post office.
*September 24, 2011 edit: So after 3 months, I finally realized that it only costs P13 to send postcards internationally and P7 locally. So if you're paying more than P13, you're me a few months back--ripped off. Anyway this post is here. Too lazy to take it out.
Makati
Ayala Avenue
Makati Central Post Office (Philpost)
Postcard: P13
Greenbelt 1
Mail and Load (G/F)
Postcard: P20
Ortigas
SM Megamall
Fax, Parcel, and Print (Basement Mega A)
Postcard: P17.50
Quezon City
Diliman
Quezon City Hall (Philpost)
Postcard: P13
University of the Philippines (Philpost)
Postcard: P13
Cubao
Post Office (in front of Cubao X, near Singapore Chicken Rice)
Postcard: P13
SM North EDSA
Fax, Parcel, and Print (Outside The Block)
Postcard: P16