Keep in mind that you will need time to address your invitations, ask family and friends for help. While doing it yourself is the most cost effective way. There are other alternatives you may want to consider: the invitation comapny may provide this service for an additional fee or send the envelopes to a calligrapher. A fun alternative would be to learn calligraphy yourself, there are many books and materials readily available.
Even though selecting your invitations may seem like a daunting task with all the possibilities available, giving yourself enough time will prove beneficial and a money saver.
It is never too early to start shopping around!