All customs details for services purchased through Parcels Online are filled in during the booking process online. You don't need to print them or attach them to your parcel.
Custom details are mandatory for all gifts and goods sent outside the UK from England, Scotland or Wales. If posting from Northern Ireland, they're only needed for non-EU destinations, except the UK. They help local customs authorities identify if items you're sending internationally are allowed into their destination country and calculate any duties or taxes due.