Always check with the post office when designing a custom postcard, envelope, parcel box, or other mailer!
While designing a brochure for a local non-profit here in Ashland, I found myself in a swamp of postal regulations. One panel of the brochure will be perforated so that potential supporters can detatch it and mail it in. Had I not checked the postal regulations, my client would have ended up with 500 unusable brochures.
The US Postal Service has size requirements for all mail, including postcards, detailing not only the minimum width and height, but even the minimum paper thickness. Not only that, but postcard rates only apply between the minimum and maximum size, and to cards that are within specific ratios of width to height.
It can be extremely difficult to find and wade through the information, so here is the link to the USPS size regulations. Even so, if there is any question in your mind, take a sample to the post office and ask them to take a look at it. You don’t want to pay for design and printing only to find out that your cards are unusable for your purpose.