I'm sorry, OP, this must be really hurtful.
I had a comparable situation, and the friendship never recovered. We were invited to the 40th birthday celebration of an old uni friend of mine in a huge, posh Airbnb in a remote location, which was a struggle to attend but I left our 2 unruly toddlers at home with DH and drove the 4 hours with my newborn who I knew wouldn't cause any hassle.Took lots of food and drinks as requested, and obviously a nice present for the birthday girl, went to the slightly eccentric pot-luck dinner, and stayed up as late as I could manage for drinks and games in my baby-addled state. The next day I got up early and helped out with tidying up and preparing breakfast for the party household. It was a relaxed, slightly chaotic all-hands-on-deck vibe.
Then, as I was preparing to leave, the A list guests began to arrive, in elaborate fancy dress. The real party, with posh catered dinner, sound system and DJ, was that night and the B list guests were being chucked out to make room for more interesting/highly valued friends.
The sad irony is that I wouldn't even have minded if she had explained the plan in advance, E.g "look Sunshine, we are doing this over 2 nights and the Saturday might be a bit full on for people with small kids - would you be happy just to come to the chilled out part on the Friday?" but I felt I had been misled and the whole thing showed that she wasn't as good a friend as I had thought - or at least, I was not as important to her as she had been to me.
There was an "honesty box" complete with card reader positioned at the front door, with a retrospective request for contributions to costs, but I declined. The implication that it would be somehow dishonest of me not to pay was almost more offensive than the B list thing! The whole experience left a sour taste and fundamentally changed how I saw my old friend. *
In your shoes I would decline the Saturday invitation - no need for drama or confrontation, just a "something's come up". The party will be tainted by the way your friend has treated you, and being expected to fetch and carry would really rankle with me.
*Postscript: exactly 10 years later, I get a message from her partner "We have a few spare beds left for X's 50th in Another Remote Location this weekend- do you guys want to come?"
Um...