OK so first off, you want these things:

• Proof of delivery
• If they say the package was empty, you can look for the weight of the shipping box from the logistics tracking website and then send this back in as proof the box was not empty.
• Invoice from an authorized distributor or manufacturer in PDF format, highlighting the items that were in the package (ASIN/UPC/SKU/something that matches the invoice to the Amazon listing)
• Probably need to keep your LoA handy if it is not your brand's products
• Inventory ledger report using the settings: Detailed View, Reference ID being the Shipping ID, and date range exceeding the time frame. This shows it being received then removed from the shipment.
• You want to upload these to a case Help/Get Help and Resources/Missing Items from Inbound.

The case text is kinda a basic template like this:

Hello,

Please thoroughly review ALL the attached documents. We have highlighted the missing inventory making it easier for you to review.

Please see that the required information is on the invoice:

- Date of the purchase

- Product names of the missing items

- Quantity purchased

- Full name and address of the invoice issuer

- Full name and address of the buyer

NOTE: inventory ledger shows these were received on xx date and then removed on xx date. Please find the missing inventory or reimburse us for the lost inventory.

If you get a generic response and have all of your paperwork together (make sure you have proper invoices), then keep at it on the same ticket until you have clear resolution.