If at amiami, keep those documents attached to the box. The one with the shipping/order number. Email them at customer support, send them an image and scans of the documents. In my case, it was an Optimus Primal MP 1st version. His "face" with lips mask had this ugly paint chip which can't be easily ignored. I ask only for the mask replacement and not the whole set. They shipped the replacement free of charge.
1. Keep those invoice papers included in the box.
2. Amiami asked for the number of the invoice, I just scanned the whole lot of them
3. Also send them an image of the defective part
4. Best of all, be nice and courteous
5.(I do have a long mileage history with them so this might also be a factor)