The package slip says that it is a small package or a letter (not clear which was ticked first). Here's the address of the post office: ПС София - 129 ж.к. "Младост", бл. No 509, вх. А ж.к. "Младост" 1А, 1729, phone: (02) 877-54-14. I found this info here:
http://www.bgpost.bg/files/custom/Post%20Office%20Sofia%20Last-1.pdf
Your package was delivered on December, 13th, and you'll have 20 days to pick up. The slip also says that you have to go to the cashier counter. 

Greetings from a Bulgarian in Berlin :)