Support and extracurricular activities?

My ex and I had a financial re-evaluation last winter, and the judge ruled that worksheet A applies to us. This was debated because the current custody arrangement has my daughters spend less than 123 days a year with me on odd numbered year and more than 123 on even years due to spring break. In any case, I ended up paying a lot more than I would have with worksheet B, which I can live with because I was confident that the money was going toward my daughter’s expenses. In spite of the higher support payment, my ex has continued to ask me for money to cover extracurricular activities like soccer and gymnastics. I’ve helped when I could, but I barely have enough to make ends meet and I told her repeatedly that the higher worksheet A payment is supposed to cover such expenses. Two weeks ago, she emailed me again informing me that it was my responsibility to pay half, and when I told her I couldn’t, she removed my daughter from her soccer team.
My attorney had told me that sharing the cost of such activities is normal practice for worksheet B, but the reason I pay so much more ($500 a month more) with worksheet A is because the support payment is supposed to cover my end of the additional expenses? Is there any truth to this, and if so do you know of any documentation I could provide so that she stops demanding more money?

Just an additional note:

My ex has more than enough money to cover the full cost of all the activities, but insists that I help cover the cost because “it’s only fair”.

Unless the court order requires you to kick in extra for your daughter’s activities you do not have an obligation to do so. Unfortunately she is not required to pay for the activities in full since the order does not impose such and obligation on her.