ReturnItem only allows for a single ReturnItemResponse which in turn only allows for a single Payment record.
This causes problems for orders/returns where a refund is split across multiple payment methods, because you can't cleanly create one refund Payment per ReturnItem.
I think returnItemResponse should be removed from ReturnItem and instead ReturnItemResponse should contain returnId/returnItemSeqId.
This would allow for one-to-many between ReturnItem and ReturnItemResponse. For the case where one payment is made for multiple ReturnItems it would also allow for one-to-many between Payment and ReturnItemResponse.