It's when the post office hand cancels the letters (the stamp that they received it for processing) instead of putting them through a machine. Brides will attempt this if they have embellished envelopes with rhinestones/ribbons etc, as well as when they are thicker then usual. Not all post offices will do it. We tried to have ours done and no one would do so. I think it's becoming more rare.
Don't know what area you're in but the woman all the way to the left (sorry, forgot her name) at 51st and Elliot told me she'd do mine. I've also seen a man at the Van Buren and 50th (I think) location do it.
Infertile, living childfree, advocating like a BOSS
My FMIL found a post office by her that will do it so we are going to have her send the invites out. I am not sure how much of a difference it will make though
Re: Hand-Cancelling
Infertile, living childfree, advocating like a BOSS