You don’t need a developer. Login to the admin portal and go to Sales -> Payments. edit the payment method you’re wanting to add a fee to(or edit all if you want it across the board).
The 2nd half of this page “Misc” is for fees. You can do a flat value or a percentage of order. I haven’t used it myself so I don’t know how it labels the fee item in the cart or on invoices.
My advice as a fellow small business operator would be to adjust your prices to reflect your needs. Customers don’t want to see what we call here “nickel and diming” on all their invoices. If most of your orders will have this fee, it could also be easier to adjust your prices and then provide a “discount” code for people not using a bag. It comes across as a thank you, rather than a penalty.