Amazon Add-On items are items that are sold by Amazon only as an addition to another order above $25 or 20 GBP. It is not possible to ship the Add-On item to a different address when adding it to another order which is above $25 in value.
How to avoid selling Add-On items when using DSM Tool?
The best way to avoid selling Add-On items is to avoid selling items with a price below $10.
Another option is to use the DSM Add-On detector that will set an item which was detected as an Add-On item out of stock. Due to the change in Amazon shipping policy for Add-On items in June 2018 the DSM Tool team is working on improvements to the Add-On detector which are soon to be released. Once it's released, this article will get updated as well.
How to sell Add-On items using DSM Tool?
To enable Add On selling, you can choose one of the two options:
If you are not using Amazon Prime protection, your Add-On items will simply show as in stock in the Price Monitor page and no further actions need to be taken
If you are using Amazon Prime Protection, the protector will set all Amazon items which aren’t part of the Amazon Prime program as Out of Stock, including Amazon Add-On items.

Learn more about the Amazon to eBay dropshipping here.