Addressing a large number of envelopes can be a real hassel. So why not just print them?
This can also be more work than you might expect. But I have some things that can make it easy and cheap,
You can do all of this on you home printer. I did this for our save the dates and it looked way nicer than handwriting them since my penmanship is not the best. To start select a few fonts, one should be a standard font, the other a script.
You shouldn't have any issues printing directly onto the envelopes at home.
Create a blank Word document
Change the paper size to 5x7 (File --> Page Set Up -->Paper Size)
Change the orientation to landscape
Type the address on the Word doc (play around with font, font size and placement)
Feed a single envelop (flap down) into the printer
Repeat this process for each envelop
I had to play with the envelop orientation when feeding it into the printer so make sure you have some extra envelops to work with. For my printer, I needed to rotate the envelop 90 degrees to the left.
Just remember that as long as the postman can read it, I don't care. It's just going to be thrown away and the guests likely don't care.
When we sent out our birth announcements I bought a pre-inked customized stamp, to put our address in everything.