The guy at the border 'accidentally' missed to hand me a USB key (which I am supposed to carry on from one control point to the other, and probably contains all the info already processed), what I found out at the next control point. When I came back to get the USB key, he said some things about how he can't find the image of my car in the computer, and how I should give him a tip of 5 euros for him letting me through (all in Bulgarian, so I could understand him only because Serbian and Bulgarian are somewhat alike). Not wanting to lose any more time at the border (I still had some 6-8h of driving to do), I gave him 10 euros (the smallest bill I had), and got my USB key.

It would not be that bad if I was the only sucker: I've seen a few other people going back for their "chips", and paying money to a guy sitting in a box with a sticker of "No payments are made here". I emailed the Bulgarian Ministry of Foreign Affairs, and the Bulgarian Customs, but I am not sure if that'll be of any use.

I am sort of used to fighting this in Serbia, but didn't expect it in European Union as well. I guess being in the EU doesn't help stop small-scale corruption either (and we all know it doesn't help stopping big-scale corruption, with the grand example of OOXML).