ADDRESSES: You can choose to address your invitations by hand, or you can decide to print labels. Don't let the WIC bully you into thinking your own handwriting isn't good enough, or that the convenience of labels is tacky. As for the actual addresses, here are a few tips on how to address invitations formally, accounting for a whole host of last name conventions.
ONLINE PRINTING: There's a pretty wide variety when it comes printing via online companies, so always make sure you can get a proof of your invitation. With some companies, that may cost extra, but it's usually not very much. Much like with at_home printing, your paper options will usually be limited to whatever the company carries, and each company's paper set is going to be slightly different. You can usually request paper samples for free. Here are a few companies I've used with good results: