When configuring a listing on Amazon Seller Central, the user must choose **one** of the available fulfillment channels that Amazon offers. We distinguish two kinds: Fulfillment by Amazon (FBA) and Fulfillment by Merchant (FBM).

However, Amazon suffers from a known issue of ghost listings. A ghost listing occurs when an offer is presumably sold via FBA, but in fact stores stock information for both FBM and FBA creating unwanted FBM orders.

To avoid ghost listings, the first solution was to disable stock synchronization as soon as an offer contained stock in the Amazon location of Odoo. However, if a merchant decided to change the fulfillment channel, it became impossible to sync the stock anymore. See also e7c01c7097d90e731c5408cee4d3595ed810c8fa.

To resolve this issue, we decided to use the Amazon API to fetch information about the fulfillment channel of a listing. However, Amazon doesn't give a clear answer for a given listing. Therefore, after some research, the assumption was that an offer was FBM if the listing contained a `merchant_shipping_group`, as the merchant shipping group is a setting specific to FBM listings. See also e6d620e4b200cadabb00ce37ab03289cfeb4ae58.

However, this assumption was flawed because Amazon can keep the shipping group even if the listing switches to FBA. This in turn enabled stock synchronization, leading to ghost listings.

To fix this issue we give the possibility to users to manually set the correct fulfillment channel of an offer when it becomes ambiguous.

opw-5480254

See also:
- https://github.com/odoo/upgrade/pull/9692