Because the system is fully electronic and linked to your license plate, the cameras and police patrols verify your payment by scanning your plate and checking it against the central database in real-time.
Therefore, unless for personal purposes, no print out is necessary.
Related articles:
Do I get a confirmation email for my Slovak e-vignette order?
How is the Slovak e-vignette checked?
Can I buy the Slovak e-vignette for someone else (e.g. as a gift)?
Does the Slovak e-vignette activate immediately?
Can I buy a Slovak e-vignette on weekends and public holidays?