Mel here from Shopify. Thanks for reaching out, I have taken a look and found a solution that may work for the set up you want to create.
An option would be to use an app like Product Fees & Charges. This app allows you to manage product fees on your products. With this app you can set it that the handling fee is automatically added to the cart with every product, however, the customer is also able to remove it which is probably not something you want.
Another option would be to create a shipping rate called "Handling Fee" and set the parameters so that it's added during the shipping step of the checkout for orders under the conditions you set.
Hope this information helps! You can reach out to the Guru team https://help.shopify.com/questions if you have any questions.
Have a great day!