Even offering free postage - seller pays you still have to use the ebay Simple Delivery choice of courier. For future listings you can opt out of using Evri here:
https://www.ebay.co.uk/ship/prf
For your current sale you could buy your own postage and apply for a refund for the SD label.
To get the buyer's address and mark dispatched you can open the original prepaid label (Get prepaid label > Get prepaid label and QR code). We can no longer change the tracking but would be able to add the new number to a "not received" case if that were necessary.
You can claim a refund for an unused label here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
The line about needing to be sent with a tracked service in order to receive the refund no longer applies.
Without tracking on ebay the payment would be held for 14 days.