i used www.weddingchicks.com to make a custom monogram. Printed it in 4s on cardstock, then cut to USPS specifications and size.
and here's the lovely back
i printed mine in black and white, so i could mass produce with a copier, and then went over the bottom text in red, and the monogram in aqua with colored pencils. it doesn't save time, necessarily, but it does save me from buying yet another ink cartridge!