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.
We are working on the functionality to enable Collectives to give directly to Collective in a different host. As a workaround for now, the Collective you want to give to can submit an expense for your donation.
To donate using your Collective balance, simply make a contribution and select it as your payment method during checkout.