Is it a fee for publication or a fee for reviewing?
Found,
> Once your paper has been accepted, we will confirm your eligibility automatically through the eRights system, and you’ll get to choose your Creative Commons license (CC BY or CC BY-NC-ND).
Upon publication almost exclusively.
It is on acceptance almost universally. This is why more selective journals have higher APCs. The overhead of reviewing and processing more papers when less ultimately convert costs money.