I just did some more searching to clarify this question:
When one OC Collective pays another OC Collective, what are the fees involved?
Let’s assume we have:
Beneficiary Member:
Hypha Worker Co-operative (a Collective)
hosted by Hypha Worker Co-operative Inc. (a Fiscal Host)
Meet.coop’s Central Account:
Meet.coop (a Collective)
hosted by The Open Co-op (a Fiscal Host)
Hypha wants to send €70 to the Meet.coop Central Account via OC. Currently, Open Collective does not support this well:
If you have a Collective with funds in its balance, you may use that as a payment method when contributing to another Collective within the same Fiscal Host. There are no fees for this method of donating.
Except we are not under the same fiscal host 
We are working on the functionality to enable Collectives to give directly to Collective in a different host. As a workaround for now, use the following process:
- Contact the Fiscal Host backing the receiving Collective and ask them to submit an expense to the giving Collective for your donation;
- The giving Collective will approve the expense and pay out the receiving Fiscal Host;
- The receiving Fiscal Host will then donate the money to the receiving Collective.
So this means Meet.coop’s host, The Open Co-op, will submit an expense to Hypha for €70, which Hypha will approve. Then The Open Co-op will donate €70 to the Meet.coop Collective.
In this case I think there is little fees involved as Hypha can direct deposit to The Open Co-op over TransferWise to pay the expense, but then I don’t know if OC takes anything when The Open Co-op donates to one of its collectives, Meet.coop.
Would be useful to invite someone from OC to look at this thread.