As with anything on the internet, you'll need to do some research? Assuming you have not chosen an invitation/company already, you'll want to do some in-person research at your local stationery stores.
Look at the large invitation company books, and be critical of everything: do you like the look of the printing? How heavy is the paper sample? How long does it take to place an order, and then how long before you receive a proof? How long between approval of the proof and delivery of the final invitations? Finally - how much will it cost for the invitation company to do everything you'd like? Can you choose which enclosures to include? Or is it a standard set?
Once you have a good idea of what you like, you can either search for a dealer online for the specific invitation you like. When you find one, look for their policy page -- what are their policies for timing, delivery, ordering? Do they have a secure ordering site that's been verified as "Hacker-safe" by a reputable agency? Or do they only take orders by phone and fax?
Some invitation companies, such as my own, are very small -- that doesn't necessarily make them any less reputable, but you might want to take some additional things into consideration: how professional is the site? Is there a binding contract (particularly if you're looking for custom work)? Again, you'll want to look into policies for timing, delivery, and ordering. With a smaller company, I would also suggest finding a phone number and calling and speaking with the owner of the company. Are you comfortable with this person, and their knowledge and level of professional service? If not, consider working with someone else.
When it comes to the enclosure cards, you can always do them yourself. However, there are advantages to having them printed at the same time as your invitations. These advantages are that the papers will match exactly -- there won't be any difference between the color of "ivory" paper from your invitation vendor, and the "ivory" paper you pick up on your own. The fonts, spacing and overall look and feel will be more cohesive if you have your invitation vendor create the additional enclosure pieces, as well.
Finally, I always recommend ordering a sample before you purchase, even if you must pay for it. There is nothing more valuable than being able to see for yourself if the paper is heavy enough, the type of printing is what you'd like, and the color is what you expect. All of those things are difficult to represent on a monitor.
There are some that considering sending cute black & white postcards for wedding invitations. Just regular black & white postcards that can be purchased in retail store, and then they can write on the back. You can pick out what style of black & white that you would want. Go to a store that you might thing they have it and have them order them in bulk.
It's unconventional and fun! There are a couple of things you should consider (if you haven't already), though, if you decide to go this route:
First, because it's such an informal invitation, your guests will assume it's an informal wedding. If you want everyone to show up in black tie (or at least in business casual wear), you'll have to be pretty explicit about that in the invitation wording, or find a way to let everyone know by word of mouth.
Then because you obviously can't include anything with the postcard, you'll need to have a way for your guests to respond as to whether or not they'll be able to come. If your guests are very internet savvy, then a web address (you'll need to set up a webpage to take responses) or email address ought to do the trick.
If there are a lot of senior citizens on your guest list (not to stereotype, but many older folks are a bit techno-phobic), you might want to provide a phone number for your guests to call and rsvp instead. There are "1-800" phone number services out there especially for wedding responses...I haven't used them, but I've heard of them. It would be a nice gesture to provide a toll-free number for your guests to call and let you know, so they won't have to pay any long distance charges.
Other than those two things, I really don't see a problem with the postcards. It's unusual that anyone would receive a hand-written invitation anymore, and that's a really nice, personal touch.