You can fulfill only the Zendrop products in a mixed order and skip the non-Zendrop items. Once you mark an item as not linked, Zendrop won't try to fulfill it in future orders.
Here is an example of a mixed order in Shopify:
Here is how to handle it in your Zendrop dashboard:
Go to your Zendrop dashboard and click Orders.
You will see a small alert icon right next to the order. Hovering over it shows:
Click the order row to expand it, then click Don’t Link This Product. Zendrop won't try to fulfill this item in future orders.
Click Fulfill Order to complete the rest of the order.
Go to your Zendrop Orders page to review mixed orders and apply these steps as needed.