We need to make a change to this API call, because of a limitation on the paymentsystem service that was just identified, and will not be fixed for release. Ash is implementing the fix in the payments extension, but the checkExisting call needs to take two separate parameters, id AND sku, and the documentation needs to be updated to reflect the following:

Only one of ID or SKU needs to be provided, and if both are provided, the ID takes precedence and SKU will be ignored.