I hope someone can answer this for me. I just "adopted" a solider and I am trying to estimate my postage costs for letters and packages.

Is mailing to an APO address considered domestic mail, or international?

Any advice you can offer is greatly appreciated!

It's domestic, but you may occasionally need to use a customs form.  But the postage cost is the same as for domestic.

Letters will be the same, based on weight. 44 cents for the first ounce.

Packages will need a customs form. Pick up a set of the flat rate boxes. The large one is discounted $2 for APO/FPO addresses. You can check at USPS.com to figure out how much weight will save you money to use the flat rate instead of a regular box. (If it fits, of course!)

Sometimes it's actually cheaper to use the prepaid Priority Boxes if you are mailing stuff besides books.  Becqause you can mail whatever fits in the box at that rate without worrying about weight. 

Thank you all so much!

Also keep in mind that the packages are going overseas so the time for it to get there can vary and be long.  Packages going to Iraq and Afghanistan normally get there relatively quick (a week or so) but too other locations it can take longer.  If you're mailing books I suggest you seriously consider sending them First Class instead of Media Mail.

USPS has a special flat rate box for APO/FPO addresses, and it comes with a special discounted rate :) Definitely the way to go if you are shipping anything besides books.