Hi @smiths. This sounds like the perfect use of Service Charges. You can set those up from your online Dashboard > Account & Settings > Business Information > Service Charges.
Create a service charge “Shipping/Handling Fee” and define it as needed. At the bottom is an “apply automatically to Square Online orders” which you can toggle on. Don’t forget to specify which fulfillment types (Shipping for you, I would guess) this applies to.
In the future, this service charge will be added to all Square online orders that use the fulfillment type you specify.
Let me know if you have any other questions.
