
Stamphil20 !
The information gaved by vayolene is accurate. The image in the center of the stamp is one of the emblems of the Bulgarian Post used since 1879 ,but here you could see it in it's socialist variant. These could gauge you when the stamp was issued. The value -2 stotinki and the design of the stamp are indicating that it's most probably issued after the currency reform in 1962. Perhaps the stamp is also ,from 1960s or early 70s. The cancel is not very clear, but part of the inscription spels GARA or Station,maybe used at a small village?
These stamps were most frequently used as additional service charged on registered mail and parcels in the country of it's origin and a broad.
They were not used ,for everyday correspondence,tath is why they are not so common and as far as I know there not described either in Bulgarian post stamps catalogs.
I don't think you could find many images of these stamps at internet, because now there are only few for sale in Bulgaria. One is a collection of covers with these stamps with the higher values - 50 stotinki and 1 lev on them and a other used stamp just like the one vayolene posted.
Stamps ,from bigger value and on cover are more highly valued by Bulgarian collectors.