The object is to avoid the Post Office's Automatic Cancelling Machine.
Follow the suggestions below, and hope for the best.
With security as it is, there are no guarantees.
You may ask the clerk at your local Post Office to either hand cancel your envelopes or allow you to use his stamp to hand cancel them yourself. If the Post Office is especially busy at that time, please be considerate and ask if you should return later. If you call ahead, you will be advised of the best way to proceed. Some Post Offices will comply, some will not. Try a small Post Office.

Once hand cancelled, ask to have all invitations placed in a separate tray on their way to the main Post Office in your city, and enclose a full page note that says, “These beautiful Wedding Invitations are already hand cancelled. Please be careful not to place them through the automatic cancellation machines.”  
Click here to print out this note.

Some things to think about when ordering your invitations:

Smaller rectangular invitations travel better.
Oversized and square invitations may be cancelled with magazines.
These definitely need special attention.

Have invitations sent to main Post Office with the note above in a separate tray.

If you have a large number of invitations that will be mailed to one community, take them to that Post Office and ask if they can sort them without sending them downtown first. They do not have to do this but might...that's it and good luck with the mailing.
 

 Joan Breman
author of "ADDRESSING INVITATIONS PROPERLY"