Single prints and small runs are usually produced by digital print.
There are two main technologies here: direct to lens print on UV machines and direct to lens printing using HP Indigo machine.

Direct to lens method is used where the edition varies between over a dozen 
and hundreds of invitations, postcards or similar prints.

There is also home-made alternative: Inkjet print on photo paper method and lamination to lens comes in play with proof of concept or some artistic prints. The disadvantage is sensibility to mechanical damage and humidity.