Here is how I did it;
5 months before the wedding: I made sure my excel sheet was free of errors. I missed a few which caused me to go back and fix after printing ( Lesson learned- order extra envelopes). I then did a basic mail merge to two separate documents, inner and outer envelope. I also perfected the color I wanted to use. I used a dark brown instead of black due to the fact that my invites were dark brown and dark red. I downloaded the font from my invitations. I used this website, but there are many many others.
4 months before the wedding: My husband (then fiance) made envelope templates. I had him cut out about 30 pieces of paper that were the exact size of the envelopes so I could figure out how to feed the envelopes, how I wanted it centered. This was a great idea because it probably saved us from ordering even more envelopes.
After I "mastered" the technique, I started printing the outer envelopes. I printed them in alphabetical order and did it one at a time. For some reason my printer had issues with multiple envelopes. If you have lined envelopes be prepared for this issue. It took some time, but I hand fed each one. It took me about 2 evenings to do it. I kept them in alphabetical order in a box.
I then did the inner envelopes. My printer let me do about 10 at a time which saved me time and I did the inner envelopes in one evening.
This process caused me to pull out about half my hair (good thing I have lots of hair). For some reason I had a huge smearing issue that only happened with the outer envelopes (again, I think it had to do with the thickness). I tried everything from cleaning the heads between each print, manually wiping the ink compartment. In the end I could not figure it out, but it only happened sporadically which didn't actually cause huge issues besides for making me slightly insane.
After I had everything printed and in order my mom, sister, mother in law, and I started stuffing. We had a good system going. My sister numbered the invitation with an invisible ink pen, my mom put the invite in the first envelope. I stuffed them in the outer envelope and checked it against the list to make sure the inner and outer envelope matched, and my mother in law sealed and stamped them. It took us about 5 hours to do the entire thing. We did it the evening after my bridal shower.
3 months before the wedding: Everything got mailed out! My mom actually took them to the post office and had them hand stamp each envelope, she wanted them out of her house because they were sprawled out all over her dinning room table, ha.
The printing was mostly a one person job especially since I am pretty crazy and really need everything to perfect. I realized if I had someone helping me I would have made them crazy. The stuffing was made so much easier and more fun by the help of others!
In the end I had to order more envelopes because of the ink smearing issue, but the total for printing 315 invitations was about 60 dollars including ink. I would say DO IT if you already have a printer and some patience. It saved us a ton of money (especially since I had so many invitations).
Confession: I actually thought my mom would hand address the invitations because she has fantastic handwriting, but due to the fact that I am a perfectionist after a few envelopes I decided I really wanted the precision of exactly the same writing on each invite. However, my mother had a great point at the time that I wasn't hearing. Only a few household really get more than one invitation so by in large no one really knows what everyone else's invitation looks like. Furthermore, it isn't a detail most people even notice. However, at the time this advice was lost on my OCD bride brain.
So tell me how did you address your invitations?