In Mongolia, even in the capital city, there are no street addresses. When you are looking for a place, you are told directions based on what it is near. This means you have to know where everything is and what it is near. Sharp learning curve!
I now have a mailing location that will work for the US and other countries. Typically people use their work address. Mail from the UK takes about 2-3 weeks. Mail from the US takes 3-6 weeks. I recommend against putting a value on packages. I have had packages held ransom by the post office. One package of food they wanted $60 for me to pick up. A Mongolian friend helped me negotiate the price down. That being said, 95% of the time Mongolian mail is great, but do not post things unless you are OK with it not arriving. There is no assurance, once it gets overseas.
If you are sending a package from the US, the cheapest method is a USPS flat rate box or envelope (weight is not considered!). Post offices give them out free and they have a few different sizes.