When mailing precious parcels, such as expensive wedding invitations, all the books say that it's best to hand cancel them. Canceling is the ink-stamp used to note that a postage-stamp has been used. Typically this is applied by a machine that may be rough on your envelope, but hand stamping may prevent damage.
Having done some research myself, I don't recommend hand canceling to my clients. Though I hand canceled each item, they were still run through machinery that bent the edges of my envelope. Now I tell my brides to use outer envelopes instead. Even though they will still be run through the post office's machinery, the inner envelope and the invitation will be protected by the outer envelope.
Some brides will still feel greater peace of mind by hand canceling. To hand cancel, try to visit your post office during the least busy time (I was told Wednesdays and Thursdays around 2pm are typically the least busy). I've heard many stories about being denied the right to hand cancel, but the post man told me today that as long as it's not very busy, they'll allow it. If they deny you, ask them when a better time would be to come back, but don't get deterred. Come back again with a friend, so you can get them done in half the time.It shouldn't cost any extra to hand cancel, but you'll have to have the correct postage on the parcel first. Then you take the ink-stamp and...stamp your parcels.