InvalidInput: Invalid input. Requested items can't fit into any existing package, and exceed unshipped item quantities
There are 2 different scenarios in which this error might get returned. Both scenarios will result in a different error getting returned first and on a reprint attempt will fail with the above error. The original errors and resolution steps have been described below.
Upon requesting a shipping label from Amazon, an error has been returned: 'Unable to convert the received label to ZPL format'. All consecutive retries will result in 'Requested items can't fit into any existing package, and exceed unshipped item quantities' error.
This means that the consignment got created on Amazon end with the first label print request, however the system was unable to produce a label for it due to the fact that label was returned in PNG format, but the user setting has ZPL format specified in Shipping > Integrations > (New) Amazon Shipping > Printer > Template.
In order to avoid seeing this error, please go to Shipping > Integrations > (New) Amazon Shipping > Printer > Template and select PNG as the label format. This should allow you to produce new labels via the integration.
The existing label that has already errored may need to be printed directly via Seller Central.
Upon requesting a shipping label from Amazon, an error has been returned: 'InternalFailure: We encountered an internal error. Please try again'. All consecutive retries will result in 'Requested items can't fit into any existing package, and exceed unshipped item quantities' error.
Despite the fact the Amazon returns the InternalFailure upon the original print request, it still generates the label on Seller Central. However due to an error getting returned instead of a successful response, Linnworks sees the shipment as incomplete and attempts to create another label on a reprint attempt. As the consignment already exists on Amazon, it does not allow an additional one to be created, which is why the second error is returned.
Users are advised to print the generated label from Amazon Seller Central directly, as it would not be possible to produce it via Linnworks due to Amazon API specifics.
The InternalFailure error has been reported to Amazon for further investigation, so for the time being the error might still get returned intermittently for shippers.