I probably was not careful and had a trial subscription or something, and apparently I was charged a lot every month. I learned about it only after I changed my credit card and stamps.com started to send me emails that they cannot charge the subscription, but what's unusual is that they still consider it me owning them money (normally they would not start subscription if it not pre-paid), and keep asking me to pay what "I own them". This is the first time seeing such weird interpretation of subscription service. This is very dangerous to deal with them, stay away!