Jamie here from Shopify. The three places you have control over your pricing is on your product pages, your tax rates, and on your shipping rates. You can certainly adjust these rates accordingly, but we do not offer the ability to add any additional percentage based fees at this time. For example, I have seen merchants use the shipping rates section to display rates that were titled "Shipping and Handling", or "Shipping and Service Fees" as an example. You can also bump up your product prices a small amount.