I just used Flywire to pay school fees in the US. The service charge is 3% if credit card is used. It is actually cheaper to use bank transfer. In my case, I choose to pay in USD as I have USD in my bank account in Hong Kong and Flywire charges USD 30 fee. There is also the standard remittance charge for the remitting bank in Hong Kong.
If you choose to pay in Hong Kong dollar, Flywire uses their exchange rate to determine the amount of HKD to be remitted to them and there will be no additional service charge. According to Flywire, the rate that it uses will be better that the bank rate. I have not tried this route and not sure if this is the case.
I did it at the branch, not by internet banking. The USD 30 is charges by Flywire. The remitting bank in Hong Kong will charge its usual outward remittance fee.