When designing card mailers, make sure you leave enough room to accommodate the barcode.
When choosing paper for self-mailers or cards, make sure the color of your stock is light enough that it will not interfere with postal barcode-reading equipment, which could increase your postage cost substantially.
Paper with dark flecks or patterns, background screens, etc., can interfere with barcode-reading equipment. Keep the address block free from background clutter when possible to realize full savings.
Card- and self-mailers, both in printing and processing, are generally less expensive than pieces inserted into envelopes. Plus, they are easier for the recipient to see the important information.