My parents split over 20 years ago. Now that I am getting married they are both generously chipping in to pay for my wedding. Fi and I wanted to keep the wedding on the smaller side so we can spend time with the people we are really close to. We asked all of our parents 3 months ago who was on their "must have" list to be invited. We took their must haves and combined them with ours to make sure we were under 100 people. We trimmed some of our coworkers and friends we hadn't seen in years to keep us under 100 people. I asked my mom and fi's mom if we could remove a few of their folks to stay in our range and they both agreed it was fine. Also note when we hit 101 people, our venue rental price goes up $1000.
Last night I get a text from my step mom: "Have you determined how many will be seated a table?"
Me: "No idea. I would imagine 10 but we haven't gotten that far yet."
Her: "K we would like to have one table for friends. If that puts you over your count your dad and I will take care of it. We decided we would like to share the day with some of our friends. K?"
Me: "That puts me in a hard spot. I told my mom and Mike's mom they could not invite anyone other than family due to our space restraints. Also it's an extra $1000 plus food and drink cost if we go over 100 people. Let's talk about it in person next time we are together"
Her "Your dad is with me. We will cover that cost too."
My initial response was irritation. Not only do I wish these people would have come up when I asked for a count three months ago so we could start planning, I wish my DAD would have talked to me about this, instead of my stepmom sending me a text message. Also they want TEN people? That is 10% of my guests list. 10% of the people at our wedding that we don't even know. Rant over

After I've had a day to get over my grumpiness, I am trying to think about this rationally. How do I handle this? Because he is paying for half of the wedding and offering to cover the extra rental and food/drink fees is he allowed to invite anyone he wants? If he invites ten friends do I need to let my mom invite the friends she took off her list when we asked her to since she is paying the other half? I want to be respectful of my dad's wishes but I am afraid I am going to offend my mom and fi's mom if my dad shows up with his own posse.