There are a couple of other options. You can use a flat rate padded bag and stuff a box inside for $6.10. The only problem is that Post Offices are not allowed to keep them in stock. You will have to order them from USPS.com. They are free and shipping them to you is free. You will need to open up an account, with a username and password.
If you are mailing larger items to zones 1 through 5, you can use the regional rate boxes. There is a weight limit, but I have been mailing cards to Colorado for about $8 while a medium flat rate would cost around $12. Once again, Post Offices are not allowed to stock the Regional Rate boxes, so you will have to order them from USPS.com. You can save even more by printing the postage yourself. At this time, there are 3 different pricing tiers, with 2 styles of boxes for each tier. Regional A, Regional B, and Regional C. The Regional C's will be eliminated on Jan 17th, so don't order them.
Up to date pricing is available at USPS.com, then click on the link for Postal Explorer.
http://pe.usps.gov/text/dmm300/Notice123.htm#1083767
Click on the link for Priority Mail - Commercial, and scroll down to Regional Rate.
This should save you some bucks.
Rick