All memberships, all services at the AO3 itself are free, and will apparently remain so. One pays for an OTW membership through PayPal, or with an international credit card, but an OTW membership and also donations to that org are completely separate from the Archive. As long as that remains so, as long as the OTW itself doesn't directly host so-called objectionable content, then the credit card powers that be cannot "squeeze" the OTW.

–N