How can I adjust the handling fee % based on retail price?

[lastanvil]

Hi Everyone, looking for advice on how to adjust the handling fee % to track the retail price of the order, rather than base it off the carrier rate. Currently we prefer to adjust handling fee to be a % of the purchase order of a customer + flat fee, but Shopify only allows us to change % based off carrier rate of shipping a package.

We are currently using a third-party to display live rates. 

If anyone has helpful info, I’d love to get in touch! Thanks.

[TaskHusky]

I'm not aware of away to change the handling fee to be based off of anything other then the carrier rates. You could achieve what you are looking for using an app like https://apps.shopify.com/magical-mandatory-fees. 

[lastanvil]

Follow-up question, are there any apps that would not have the custom fee be an entire block of our app that customers must add to the cart?