This is a member to member forum that eBay rarely read

The many problems with Sendle are widely known

If you as a seller choose to use them, that is 100% your choice

If keep using them, that is the risk you take

You can't say 'As a seller committed to providing a high standard of service and ensuring customer satisfaction'  but then continue to use Sendle,( a courier broker, not a courier) who uses the cheapest possible option 

I am not sure how you expect eBay to 'investigate' simply because it is one of the shipping options listed

The resolution would be to stop using a 'service' you have had constant issues with, and who have literally hundreds of warnings here on the member forum alone about (let alone other review websites)