Hi all! I just received my wedding invitations in the mail, and I had them printed in double thick cardstock, so they are super rigid. My invitation company told me I need to send them as "nonmachinable." Any idea how I do this? I know it will require extra postage, but is it as simple as just putting the extra postage on the envelopes and taking them to a post office? Or do I need to include some sort of marker on them that they cannot be put through a machine? The USPS website has not been very helpful in trying to figure this out (or, I am just not looking in the right place!). Thank you so much in advance for your help with this!