The wonderfully observant Colleen pointed out that I'd made a mistake when listing the maximum dimensions for a post card (mailed at the US post card rate)
The maximum size is 4.25 x 6 inches.
Cards that are bigger than this will need a first class "forever" stamp (and possibly more depending on your size)
I've updated the dimensions in all of the current swaps so they are now listed correctly.
Hope I didn't throw anyone off!
Thanks so much to Colleen for catching this.