How should a shipment of several parcels containing personal used items, be documented to ensure that the accepting customs is informed that this is intended as a tax exempt delivery of used personal items?

How does the post office assist the non professional sender to comply with this requirements?

My buyer of some used toys is having difficulty figuring out what forms need to be filled out for Customs to clear before he pays the tax. He says he needs a private service to help him, which he cannot afford, so wants to return the items back to me. However, I would be very happy to help him and complete the forms for him - it is very simply that he has bought some used toys from me for his personal collection, and I have all the information for any forms on his behalf. Can someone help us?

I have a package, LP186192577GB, that latest status has it in customs , needing to pay duty tax. However the receiver has not received any notice. Is there a way for the receiver to get notfitication via text to personal phone or mail to shipping address to pay duty tax?