I’d be extremely concerned by those grades as well. The As in PE and cooking don’t count for much; computer is good, provided she’s learning programming and not how to type or basic HTML.

That said... while I think you’re right to not pay for a venue for a big party, you should consider how much value you place on the relationship with her. Your stance on earning things is right, imo, and a great thing to teach children - but relationships with people we love aren’t always so transactional. You could compromise by taking her and 2 or 3 friends out to a nice dinner, not because she earned it, but because celebrating this birthday is important to her and she is important to you. That sort of gesture is worth a lot, but only when it is sincerely, freely given - not transactional. Good luck!