These days, almost all mail is routed through automated sorting machines, which take pictures of the front and back of the envelope, and try to figure out the correct postal code/zip code/post office to send it to, based on those pictures. The machines are really very good at their job (99%+ of the letters get to the right destination)… But there are things that can throw them off.
This doesn't mean your wedding invitations won't get there, but it may take extra time if your wedding invitations have to be hand-sorted, or your wedding RSVP cards go to the wrong post office first.
Once upon a time, I spent about a year working on improvements to the code for postal address recognition (in a couple of different countries; it's customized a lot for each country, but the basic ideas and system are the same).
Based on that experience, here's my advice to help your all wedding invitation mailings get exactly where you want them to go, as quickly as possible…
Check the rules and prices your postal service has for mailing before you buy supplies or postage
Nothing is worse than getting everything assembled, taking it all to the post office, and finding out that you don't have the right postage, or your envelopes are too big/small/can't be covered in chunky glitter.
Use standard practices on your envelope
Put the address where it's normally expected to go, some sort of stamp where it’s normally expected to go, and your return address in the normal corner, or on the back of the envelope. Now is not the time to get creative!
Use postal codes/zip codes
They help send your letter to the correct city on the first try, and should be at the end of the last line of the address, or on the bottom line of the address all on their own (or wherever your country conventionally puts them).
Use a LIGHT envelope color
You don’t have to stay with plain white, but it should be a light pink, blue, green, cream, or whatever other pale color you want. Avoid anything shimmer-y or with reflective bits in it.
You want lots of contrast between your address and the envelope. In addition, some postal services stamp bar codes on the bottom of the envelope to help route it to the right place — you want those to be visible on your wedding invitations!
This means that as gorgeous as envelope and addresses like these are, they may have deliverability issues!
Keep the numbers and letters in the postal code/zip code separate from each other
Be careful if you address invitations by hand, and try to keep the numbers and letters in the postal code/zip code separate from each other. You don't want them to overlap, and don't get too creative with your calligraphy in that section.
Put all the shiny creative things your heart desires INSIDE your envelope, in whatever color you’d like
You can even do an inner envelope if you want to have the full effect of a silver envelope addressed in gold foil (I got these silver envelopes and loved them). This will also let you have a nice envelope that doesn't have those postmarks and bar codes, if that's important to you.
Don’t get creative with ink color
Black is great. Dark blue is great. Dark purple is probably okay. Anything else is not going to be picked up by the mail scanning cameras as well. It will probably still be fine, but it increases the chances of problems. Red, purple, orange, or gold can be especially troublesome.
Don't use silver, gold, glitter, or reflective ink or labels on your envelope
The mail scanning camera, which takes an image of the envelope for processing, will not read it properly because the light illuminating it will cause strange reflections, and you’ll end up with half your invitations routed to zip code 10000 instead of 40000 because there was a bar of light over the first part of the zip code.
This mistake will likely be corrected by a human eventually, but it may delay everything by a week or so until your wedding invitation is manually reprocessed.
Don’t use too many designs on the outside of your envelope
…Especially right behind the address! That pretty checkered design or watermark could turn MI into MT when it’s read in by a computer, and send all your invitations on a trip to Montana. Definitely don’t do anything creative with blocks of text that closely resemble addresses on the outer envelope, because the computer processing the addresses has never read Shakespeare, and can't tell that quote from a city name.
Don't put anything which could get caught up in machinery
They can also get caught on other envelopes or pulled off on the outside of your envelopes. Sadly, that means stuff like these adorable wax seals probably aren't smart:
Don't get too creative with the font
The systems will recognize any standard font, but the more exotic ones — especially the ones designed to look like fancy calligraphy — can confuse them. Especially bad in the abbreviations and postal/zip codes, where GA and OH can start looking similar more easily than you'd think.
If you want to save money:
Check the rules for basic first-class mail in your country before you purchase supplies. In general, odd shapes, anything too large or too small, and anything overweight will cost more.
Now, if you're saying “OMG! I already bought my black envelopes and silver pens! I've had my heart set on them since I was twelve!”
Relax, your invitations and RSVPs should still get where they're headed. You might want to allow for some extra transit time, though. Worst case, they take a side trip to the wrong post office, and a human being there figures this out and sends them on to the right person, or back to you.
…You did put those return addresses on your invitations, right?