Envelope liners are typically $$$$ but the talented team at Cards and Pockets will let you pick out a pattern, choose the perfect color to print the pattern in, and then create your envelope liners for a cool 45 cents each! Check out these examples below. If you don't like any of the 15 pre-designed patterns, you can work with their print department to have custom-designed liners like the gorgeous fuchsia and turquoise one below. Don't forget that you should also order your envelopes from them to be 100% certain the liners will fit perfectly.
All images courtesty of Cards and Pockets.