I recently had a conversation with my local U.S. Postal Service mail design analyst, a gentleman I've worked with before. He was kind enough to help me through some of the basics that are everyday concerns for card makers.
Perhaps one of the most important things to learn is that card shape plays a major role in how much postage is needed. Mail pieces of one ounce or less that have the proper aspect ratio are machinable and the least expensive; i.e. they require a single first-class stamp. So, what's an aspect ratio?
An aspect ratio is a calculation to determine whether a mail piece fits size requirements. The aspect ratio is met if length divided by height is at least 1.3, but no more than 2.5. (An envelope's length is considered the side that is parallel to the delivery address.) For example, a 6" x 6" card does not meet the aspect ratio; 6" length divided by 6" height = 1. A 6" x 6" card fails the aspect ratio and requires postage at the non-letter rate. (Because the rate may change from time to time, it is best to check the amount with your post office before mailing.)
A common finish size for card makers is 5.5" x 4.25", or half of an 8 1/2" x 11" sheet of (folded) card stock. A standard A2 envelope for a card of this size measures 5.75" long by 4.375" high. The calculation is 5.75" length divided by 4.375" height and the aspect ratio is 1.31. Perfect! (See example A below.)
But not so fast! Take a look at what happens to the aspect ratio if the envelope is turned on its end. (See example B.) A machinable (one-stamp) card suddenly fails the test (0.76) and requires extra postage.
The second consideration for postage is thickness. We card makers love to pop up our designs with dimensionals, but we need to be careful to stay below 1/4" thick. Anything more also requires additional postage.
Finally, our card thickness shouldn't vary more than 1/4" at any point. Designs that rise and fall more than that across the entire surface are not "flat" and therefore not machinable. They, too, require additional postage.
Here are a couple of helpful web links to the U.S. Postal Service:
A Customer's Guide to Mailing
Questions? Leave a comment and I'll try to find an answer for you. Meanwhile, happy mailing.
Thanks for visiting, and I'll see you again soon. Peace!