Your customers can pay either with a credit card or an invoice. If the credit card option is chosen, your client will be immediately charged after the session and you will receive the amount in your bank account within 4 days.
If payment is to be made by invoice, the customer will receive an invoice indicating the amount that is owed and will need to be transferred to your bank account within 7 days. PayPal payments will be available as an option soon too. As an expert, you can specify in your dashboard which payment methods your customers should be able to make.