Normally, you bill your customers on an hourly basis. If you do your support remotely, you have a great opportunity to make precise calculations based on your connection history statistics. Here, web-based solutions are a good choice, as they often allow seeing your sessions history for any given period, with details like remote machines’ IP addresses.
While today many payment options are available with software vendors, it's sometimes takes quite some time to make a decision. Let's see what is currently offered and what the possible advantages and pitfalls are.