If I am unable to help as much as she wants me to, may I tell her that I can't at the moment, but I will when I can?

Follow the court order. If it is silent then you do not have to do it. Use of alawyer is recommended.

If your MSA and final judgment do not obligate you to participate in the cost of bus transportation, then you do not have a legal obligation to do so, and could certainly explain to the ex that you are unable to do so at present but will do so as soon as you are able. As for a moral obligation to participate, that would be up to you. You should also bear in mind that if the bus transportation is truly necessary for the minor children to get to school, and is not already part of the school-related items covered by your MSA or final judgment, then depending on the specifics of your situation, the ex may potentially be able to file a modification action seeking to include this as part of the children's support contribution. Consult with an attorney about the specifics of your situation and your options.

You know what? The very best thing to do is take the MSA and FJ to a local family lawyer and discuss this. You are not required to pay for the bus transportation. Is that a valid expense or is it free? Is she trying to dupe you into giving her more money?

If it is not in the marital settlement agreement you probably are not responsible for paying your half. However if she goes back to court and tells the judge you're not paying half of the children's school transportation the judge might amend the marital settlement agreement to require you to pay at least half. It might be a pay now or pay more later situation. The risk is yours to take if you go back to court.