There is nothing I hate more than opening my email to find that I have just sent a payment off to a service that I no longer wanted. If you use PayPal you are going to come across a product or service that will be subscription based.
The problem is when you forget about it, no longer need it, and then WHAM, you pay for it again.
How much can this cost you? Well, each subscription has a billing limit, a top level that can be charged. I have seen this limit as low as $100 and as high as $375,000.00 per month.