Having your third cousin's twice removed grandmother on your step father's side show up at your event can be as easy as adding "... and family" on the invite envelope. To avoid that, be as specific as your budget will allow you to be when addressing your wedding stationary.
Using inner and outer envelopes for your invitation eliminates a lot of confusion on your guests' part and is a tradition rooted in history. Back in the days, when wedding invitations were delivered by hand of the bride's footman, having an outer envelope made sense - the outer envelope with guests' names and address spelled out was removed and the inner, pristine envelope was presented to the addressee. Since the invitation made its way to its destination, no address was needed on the inner envelope, just the names, therefore - no addresses on the inner envelopes. We have come a long way from hand delivered invitations (although, "Sweet 16" did bring that tradition to light), but to this day, some brides prefer using two envelopes when sending out their invitations as it is a very elegant way of starting the process of setting your guests' expectations for what's to come on the day of your event.
If you choose to use only one outer envelope for your invitation, be sure to follow the etiquette and all things proper. Crane's The Wedding Blue Book is very helpful on the subject, but if you don't have the time to buy one, and then read it, here are some pointers.
Be sure to spell out all abbreviations and your guests middle names - no St., Ave., P.O. or #, but rather Street, Avenue, Post Office and Number. If you are not sure of your guests' middle names, utilize White Pages. If your guest refers not to have the middle names spelled out, omit it.
The back flap of the outside envelope bears the sender's address only, no names, being the address of whoever issued the invitation.
Consider hiring a calligrapher - there is a number of calligrapher out there, who would love to address your wedding envelopes for you. Some are just starting out, and their prices are rock bottom; others have been in business for a while and therefore charge accordingly; few use a machine. Just find the one that is in your budget and if the budget is not willing to stretch, you can always run your envelopes through a printer. There is a number of elegant fonts that will do the job. Some envelopes, however, are heavier than others, so, be sure your printer is up to the challenge.
Zip code, on the invitation envelope, has a line of its own after the city and state.
Talk to your post office about hand canceled services. After you spent all that time and money on putting together your wedding stationary, don't let the post office feeding machine damage it or put bar code/stamps all over your envelope. Work with your post office to see when would be the best time for them to hand cancel one hundred or so of your invitations. It could be a Saturday morning, or Thursday afternoon - just talk to them and they will let you know. This isn't a service that post office advertises, but it does exist. Don't be surprised if the postal worker leaves the office to find out what a "hand canceled" service is, but be surprised if they tell you that there is a charge- this is a free service and should cost you nothing. If you hear otherwise, ask to speak to somebody higher on the "food chain".
For more help with your envelopes, leave me a comment, send me an e-mail or check outmy website. I look forward to hearing from you :-)