Advice for creating a crowdfunding web application.

We have this capstone project, it's a donation-based crowdfunding web application. Initially we planned to hold the donated funds just like GoFundMe. In short we are going to be an intermediary between donors and fundraisers. But our adviser said that we shouldn't hold the collected funds, he said that it should go directly from the donor to the fundraisers. But it came with a problem. How do we keep track of the total funds donated. And we have this transaction fee of 3% that we want to implement. For example if a donor wants to donate $100, the total will be $103, but we have no idea how to implement it. We are planning to use the MERN stack. Any tips?