Although the customer has booked a certain Service and paid for it, the actual consumption may change and increase the cost. This happens from time to time.
If available, you can require customers to pay the additional costs onsite; the more common way in ecommerce practice is, you can upload a special product/service named “$1 product”, so customers are able to pay the additional cost by purchasing this “$1 product”, if the additional cost is $10, then just order 10 units of the special product will do. (Platform commission will apply to this special order as well.)
