Adding Paypal as an optional payment method in Appointments+

Hi, I am using Appointments+ on a site offering several health services. Local clients do not need to pay up front and since most people prefer to pay after their visit, we don't want to force pre-payment during online booking. However, some services (i.e., Skype consulations) need to be paid for in advance. Is it possible to enable paypal payment for only certain services or, if not, to make online payment optional (so that local clients can reserve online but still pay in person)? Thanks!