Beauty should always be found on the inside - something our mother's have taught us from a young age. This rule applies to all your mail and note ventures too. An envelope liner provides the added hint of surprise, and complements the goods found inside.
In short, an envelope liner is a printed design which covers the inside flap of an envelope. The process purely involves creating your design, then printing and cutting it to shape. These are then inserted, folded and secured by hand.
The technique adds a lovely pop of colour to enhance your stationery (as a subtle addition, like a solid complementary colour) or acts as a design feature which adds to your stationery (think along the lines of florals, glitter, foil, or bright colours).
This is a luxe detail that is truly worth it. Once you see how the liner ties an invitation suite together you will never go back.