If you have a tracking number, you need to provide both the Tracking number and the Shipping carrier, eBay requires both.
The tracking info must be provided at the same time when you mark the order as fulfilled, otherwise this information will not be retrieved by eBaylink and not updated on eBay.
If you do not have a proper tracking ID leave it blank. Do not enter comments or n/a or no or - o. Leave this field blank if you do not have a proper tracking ID. Likewise you must have a recognised Shipping Carrier name entered not Courier or other etc.
See below the Order Fulfilment screen in Shopify:
Every few hours or so, we retrieve order updates from Shopify.
Once the order is fulfilled in Shopify and the information is retrieved, the shipping info will be updated in eBay.
Please be aware that the update to eBay doesn't happen instantly when you fulfill the order in Shopify.