On mailing day
How to mail wedding invitations
Assemble one complete invitation first, with every card and the stamped reply envelope inside, and have it weighed at the post office before you buy stamps. Weight and shape set the postage, so a guess can bring a stack back to you.
A 5 x 7 invitation in an A7 envelope is a standard rectangle. Square envelopes, rigid ones, and anything lumpy with a wax seal or ribbon go through by hand, and the Postal Service adds a nonmachinable surcharge even under one ounce. Many couples ask for hand canceling at the counter, which keeps envelopes out of the sorting machines; availability varies.
Mail overseas invitations first, since they take longest and need international postage. Mail one to yourself too, so you can see how it arrives and when.