When a customer contacts a detailer, it is important to know you business. Know your prices and why you charge them. It helps if there is a system you can use to determine what best suits the wishes of the customer and a package that fits their needs.
When detailing cars (semi)professionally, it helps to know what you can ask for a certain job. Although you still need experience to fill in certain formulas, this will give a rough idea of how to get a fair price and to calculate your expenses.