Documentation here: https://payments.amazon.com/documentation/automatic/201752090#201752090
When an API post is made with the billingAgreementID parameter and recurring is true, use the PwA SDK to make these calls: SetBillingAgreementDetails, ConfirmBillingAgreement, GetBillingAgreementDetails, and AuthorizeOnBillingAgreement. Handle errors the same way errors are handled with one time payments.
Send the donation to the queue as recurring.