Tantalizingly brief item from Focus:

‘The antiquities smuggled from Bulgaria are oftentimes brought to Germany, which is the most frequent final destination for them. Germany has the biggest market’, Rumen Danev, Head of the Customs Intelligence and Investigation to the Customs Agency told FOCUS News Agency in an interview.
In his words there are no practices for the valuables to pass Bulgaria as transit, so perhaps they are transported by other ways or there is another type of transport used.
The biggest seized package of antiquities consists of 5 cardboard boxes weighing over 50 kg that were seized at the Kalotina border checkpoint.


... in a related (and semi-contradictory) piece, from the same source:

Antiquity smuggling is most frequently made by cars and buses. The cultural and historical valuables are concealed both in the drivers’ cabins and in the different holes of the motor vehicles, Rumen Danev, Head of the Customs Intelligence and Investigation dept. to the Customs Agency told FOCUS News Agency in an interview. In his words the most frequent case is the use of courier and mail parcels. Last year there were four attempts at illegal carriage of antiquities though the customs of the airports in Varna and Plovdiv, which is not typical, he noted.
Sometimes perpetrators throw the valuables at the customs areas. The value of the seized smuggled items is estimated by experts from the museums of history in Bulgaria. ‘Oftentimes the experts take the items at the respective regional museum by filling in a special document’, he explains.