How Do I Address Invitations w/ Multiple Family Members w/ Different Last Names and same address?

It will seem strange, but each social unit should receive their own. It's not only appropriate, but makes it easier for them to RSVP.

^ They gave good advice. I'm just here to tell you to change your avatar to something other than the rings

Sorry, I just set this up two days ago and waiting on the photographer to give us our engagements, I will change the photo later this week. Thanks for the advice!

Just did the math. If it's roughly $5 an invitation (including postage and envelope), and I am sending 6 to the same house, it is $30. I have to do this to four separate house, so I am paying $180 to send to the same places over and over.

Yep. That's right. It sucks, but that is proper etiquette.

That's how it goes when you spend $5/each per invite.

Perhaps try to find cheaper invites. Ours were only just over $1. You really can find some lovely options that aren't so expensive. In order to be polite you need to send each adult/couple/family (meaning a couple with kids) their own invitation. So you either need to pick cheaper invites or deal with the cost.

You're sending six to the same house? No one that lives in the house is a couple? Each couple/unit, as pp have said, get their own invitation. One possible way to cut down in cost is to hand deliver them IF they are immediate family members. You can also do online RSVP to cut down postage costs.

My FH's adult sister and adult stepbrother both live with his mom and stepdad. We're sending 3 invited to the address, because they are 3 different "households".