Is it illegal to copy and paste an invitation onto Microsoft Word if the invitation isn't meant to be free?

Yeah, I think that’s called copyright infringement.

If you copy the work of another and appropriate it for your own use, other than “fair use” (which covers a lot of ground: critical reviews, parody and satire and other “attributed” uses), then that’s a form of copyright infringement.

But this is a broad and complicated area in law, and it’s not the kind of “illegality” that the police are going to be interested in prosecuting, at least while you’re only doing this for personal use or within a small circle of friends. The copyright owner might be interested and hostile to your intent to bypass payment for their work, but they would have to balance the costs (and risk) of mounting a court case against you against the potential lost revenue. That’s if they even noticed what you had done.

More than likely if they even discover that you have hijacked some of their work without paying them or without even attributing the work to them (and how likely is it that they’ll discover your act?), then they’d send a stern letter from their attorneys to put you on notice that they do not approve and will sue if the actions continue. (At least if you attribute it and are later sued you can claim it was “promotion”, and they might even buy that—whatever gets their product noticed might actually gain them more revenue down the line.)

But even if it’s not “illegal”, and even if you don’t get caught, and even if they don’t penalize you severely if you DO get caught, it’s tacky.