FedEx asking export proof for a repaired item I carried to the UK. What do I give them?

I had an electronic part that needed to be sent to the UK for repair. As I was going across, I took it there with myself and posted it domestically with Royal Mail. The repair is done and now they’ve shipped it back to my Irish address with FedEx.

FedEx emailed me asking for an export declaration or An Post receipt to prove the item left Ireland originally, so they can process it as a returning goods entry (tax-free).

I don’t have that because I physically carried it over.

Has anyone been through this? What did you provide to FedEx? Will they accept if I explain that I took it across with myself?

Edit: FedEx will not deliver the item until I provide them the proof. I had bought the item secondhand and I don’t know if I have a receipt for it.