Order ID or Order Item ID for Customs?

We have received an email today of a parcel (book worth about £70) to the Netherlands being returned by Evri and looking at the P2G tracking info it says it failed customs clearance. We don't know why that is as so far as we're aware we filled in everything online. However we noticed today when looking at the order that as well as the order id there is an order item id. We have no need for those numbers so hadn't paid much attention before but Amazon tell us to put the number on the parcel when sending to the EU. We had put the order id on the parcel but was it actually the order item id that we needed to put on the parcel and might that be the reason it failed customs clearance?

Thanks for any advice on this.