Hi my parcel EB762404888CN shows that my parcel was held/to be inspected by Customs, and then it was returned from Customs. Will this mean part of my parcel was seized and returned back to the seller?

I purchased a duvet set from an English retailer that I was unaware was to be delivered from China. The goods arrived on 28.4.24 and were unsuitable. I, therefore, returned the items by Royal Mail on 7 May 24 and have been tracking and chasing ever since. The parcel has not yet been returned to the supplier, being stuck in Customs in Souzhou. I now know that I should have been advised by the Post Office that a completed customs form should have been attached to the parcel, which it was not as unaware. I suspect this is why the parcel is still noted as held in customs when I track it. My problem is how do I rectify this with Chinese customs to enable return and refund to the supply. Any advice/help very gratefully received please.

Receiver’s country Customs won’t release the parcel. The last tracking status is: 境外进口海关留存待验(限制进口的物品-进口许可证丢失/不适用)
I have asked the customs to return to sender, but customs is saying that the sender has to ask from their side.
I contacted the sender, but they are saying they don’t know how to do this.
How can the sender ask the parcel to be returned? What is the process. Parcel sent by EMS China Post