The standard cost of sending a postcard is 0.40 lv within Bulgaria, while a letter up to 50g costs 0.65 lv. Postcards and letters weighing up to 20g cost 1 lv to elsewhere in Europe and 1.40 lv to the rest of the world. Get the most up-to-date rates at www.bgpost.bg.
To send a parcel from Bulgaria, it will save you hassle to bring it unwrapped to the post office. Anything heavier than 2kg must often be taken to a special customs post office (ask at the post office for information).