Hi Louise,
Well I’m not sure where the issue is.
I would rebuild the entire shop with Webflow ecommerce and allow the owner to charge the customer for delivery based on the weight of the product the client is buying.
I don’t think any client want to buy something online, then wait for the owner to validate the order and ask extra money to the client for delivery. People want to buy products knowing there is an extra amount of shipping money included in the price he pays online.
Hope that helps