Yes, since the e-vignette is only linked to the number plates of the vehicle, not to the person, you can buy the vignette for another person (e.g. as a gift).
Important: Make sure you enter the correct details of the licence plate of the person's car into the system when purchasing the vignette. For errors entering the license plate details no refunds will be issued.
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Where can I buy the Bulgarian motorway vignette?
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What different types are available for the Bulgarian motorway vignette?
How much does a Bulgarian motorway vignette cost?
Do I get a confirmation email for my Bulgarian vignette order?