You are registered as a private seller. Private sellers are limited to selling their own, unwanted possessions. They are also prohibited from drop shipping (which is a business activity) and required to use eBay's own, "simple delivery" system.
To sell multipe new items you need to register as a business seller. You can then drop ship and use whatever delivery service you wish. You will, incidentally, also be required by eBay and by law to accept returns and to publish your business address.
For more information see: https://www.ebay.co.uk/help/policies/selling-policies/selling-practices-policy/business-seller-polic...
