All or some orders from eBay are being allocated to Default shipping/postage service when an order is downloaded from eBay.
The service the customer selected on eBay at the time of the checkout is not associated with a service in Linnworks. Basically eBay service is not mapped to Linnworks service.
Map the service. And all new orders from eBay will be allocated correctly, to the selected service. Any existing orders can be allocated manually.
See documentation on how to allocate eBay services to Linnworks Service