How do I add an order items fee on specific items?

[idango]

Hi everyone,

What is the best way to add my fee on items in the order?

E.g., I saw that on upsell apps, the app charges its fee on products bought thru the apps upsells:

https://apps.shopify.com/upsellin

https://apps.shopify.com/reconvert-upsell-cross-sell