Invitation card is still the most common way of formally inviting someone to an 18th birthday part.
In your invitation, don't forget important details like
Date and Time
Dress Code, Attire
Map of the Venue
Instructions on how to get on the venue. Include directions for public transportation and private cars.
Additional instruction like bringing the invitation card to the event
Other itmes that can be included are
I think you already know this stuffs above but I'll share also a list of suppliers of invitation