Georgian post is expensive and slow, and seems like dying. To send a postcard you might have to go to one of the Tbilisi post offices, which are located away from the main tourist routs. Today there are couple mail boxes in the city, including one on Freedom Square, but we have also stumbled on a broken one. Post stamps also can be bought only at the post offices.
A regular post card to Europe will cost you 4 Lari, to the USA 4.80Lari.
An ordinary letter up to 100g costs 13.20/15.20 Lari accordingly.
A parcel up to 500 g - 49,90 Lari/66.05 Lari.
Exact rates to you country are available at www.gpost.ge