Guideline 3.2.1 (vi) already gives a lot of information.
But you have probably noted that it is likely Reviewer will ask you additional information:
Additional information shall be provided to App Review upon request.
If I understand you'll be an intermediary for other charities.
Hence, an absolute requirement is that you get a proof that they have gone through the nonprofit approval process, as explained here : https://developer.apple.com/apple-pay/nonprofits/
Nonprofit platforms that connect donors to other nonprofits must ensure that every nonprofit listed in the app has also gone through the nonprofit approval process
Topic:
App & System Services
SubTopic:
Apple Pay