[Shopify] Automatically add credit card processing fee to B2B draft orders

We run wholesale through Shopify B2B draft orders. Customers submit an order, it appears as a draft, we confirm quantities/shipping, then send the invoice. Right now we manually add a 3% credit card processing fee as a custom line item. If the customer chooses to pay by ACH instead though, we remove it. Ideally, Shopify would automatically add the 3% fee only if the customer chooses credit card when completing the order (3% of subtotal + shipping), so we don’t have to manually add/remove it every time. Shopify support says there’s no native way to do this and suggested third-party apps, as usual. Is there any way or workaround to automatically apply a payment-method surcharge like this in Shopify B2B, or is an app the only option?