In general, the labeling service only applies to products that already have an EAN code on the product packaging. Thus, an EAN must also be stored for each SKU/ASIN in the product details page. If you do not have an EAN on your SKUs/ASINs, Amazon will assume that you do not have EAN codes on your cartons either and will not accept labeling accordingly.
If you don't have EAN codes on your cartons, the labeling has to be done by you or your supplier. Usually the codes (EAN / FNSKU) are there to identify the product accordingly. If there are no codes, Amazon employees may identify the ASIN manually and post it accordingly.
If the labeling service has nevertheless worked in the past for items without an EAN code, this is no guarantee that it will continue to be accepted in the future, as it is against Amazon's guidelines.
It is also possible that such Send-Ins can have a negative impact on your stock performance from your seller account.
We therefore strongly advise against such Send-Ins.
EAN labels = "manufacturer bar code" = mixed stock - anyone can send this stock to the warehouse. The stock will then be mixed with the stock from other sellers in one storage location! (Especially dangerous with products where plagiarism could be created)
FNSKU labels = "Amazon barcode "Your stock remain separate from other stock (with the same EAN)