If you drive on Bulgarian motorways without a valid vignette you are subject to be fined. The fine is typically up to 300 BGN (≈ €150) for cars. Alternatively a compensatory fee can be paid on the spot, which is lower than the fine. You can check the compensatory fee rates here.
For some cases the fine can be appealed within 14 days after being issued. Check conditions here.
Please note that even if you do not get caught directly, a camera might have recorded you while driving without a vignette and might receive a letter including a fine (fiche) via mail.
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Where can I check the status of my Bulgarian vignette?
How is the Bulgarian e-vignette checked?