In the KashFlow API, an AdvancePayment does not have a property for PaymentMethod.
An InvoicePayment does.
When an AdvancePayment is applied to an Invoice, no option is provided to add a PaymentMethod and a default option is chosen.
Once an InvoicePayment is added there is no way to edit it through the API.
Would be great if the AllocateAdvancePaymentToInvoice method had an optional parameter for PaymentMethod, either ID or object.