eu – Avoiding customs duties when shipping back forgotten items to Portugal from the UK

I forgot my manual coffee grinder and Aeropress at a friend’s place in the UK. The items were bought in the EU (but not Portugal) and I have receipts for them. They’re missing all packaging and extra parts. Retail value was about €160, and the package will be shipped using the Royal Mail.

As far as I understand I shouldn’t have to pay anything, but how do we convince customs of this in the off chance the package is examined?

Do we indicate a value of £0 on the customs form and include printouts of my receipts? Am I overthinking the whole thing?