"It depends", as most lawyers love to say.
In both cases described below, when you first go through the checkout process, you provide your payment details and authorize a future payment, but payment is not processed right away (unless you choose to pay using Paypal and use your bank as the funding source rather than your credit card. It's a limitation of Paypal in this situation).
If you hire me for General Legal Advice on an hourly basis, once the engagement is formalized and the work begins I ask that you pay for one hour of work in advance. I will send you an invoice periodically as services are rendered.
If you hire me to review a contract at a fixed price, payment is processed once the engagement is formalized and the work begins. If you would prefer an alternative payment arrangement, feel free to get in touch with me to discuss options.
Since I prefer happy clients, I also offer a pretty solid Refund Policy should my services not meet expectations.