To have this done, just ask the post office to have the envelopes or postcards hand-canceled when you are ready to mail them. This means the post office will manually sort the items rather than put them through their machines. The machines used to sort envelopes apply pressure and with that pressure sometimes color transfer or slight damage can occur.
Looking for additional help? Visit The Knot Invitations at theknot.com/paper or firstname.lastname@example.org.