I think the issue here is that the panty party, board games and pool swim are only happening in the rental, contingent upon everyone chipping in toward the flat fee for having the rental.
If 5 people get mani pedis, each person pays for their own.
If 20 people or 1 person attend the panty party/ boardgames/ go swimming, the fee remains the same.
I can understand why the Maid/Matron of Honor would say everyone who is attending the house portion needs to chip in. I wouldn’t want to be one of 3 people paying for a house, only to have 10 people go swimming and hang out there late into the evening.
HOWEVER, they aren’t being good friends or particularly gracious about your financial situation. If only one person couldn’t afford it, I’d probably just say it was fine and ask you to chip in toward snacks. I don’t know if the Maid/Matron of Honor knows other people are likely to back out if their fees go up (if this fee is already the top end of their budget, they might), or if there are people in the group who would also prefer to stay elsewhere, leading to even MORE increased costs.
In your shoes, I’d probably go for one complete day with the cheapest activities (if I could afford it), and not go to the other. It sucks that you’re not in a career that pays well and sometimes that means you can’t afford to participate in the same activities as everyone else. If the Maid/Matron of Honor or bride was nasty/ demanding, I wouldn’t go to any of it.
On the flip side, $109 per person for whole beach house for an entire weekend is a huge steal in my part of the country, so I’m wondering if the MOH/ Bride worked to keep costs down. Even if you eat out dinners, you still have the option to snack and brunch at the beach house, bringing the price per person down quite a bit (compared to a standard hotel). I didn’t see any mention of going out for drinks/ dancing, which also leads me to think this was another concession to budget. Wine at the house + board games is definitely cheaper than a night on the town. If this is a road-trip destination compared to a flight- destination for most, the price has also been reduced. The party is also coming to your town, so you won’t have to fly/ drive far to get there. Nobody wants to feel like a charity case, so even though I suspect the Maid/Matron of Honor is an a-hole, I seriously wonder if she organized this event with OP’s financial limitations in mind. (At 28, it’s possible that most of the group has enough income for a much fancier celebration and are picking beachside boardgames as a compromise.)