stamps.com high monthly subscription was not make known to me. california

Stamps.com 

I signed up for a trial back in December 2014. I was not aware of a monthly subscription program. I was not able to install the software and did not use the service at all. I did receive a package of phamplet but nothing alerting me to the fact that I have signed myself up for a monthly subscription of $15. I did not receive any email on the that the trail is over nor a statement of the monthly subscription. 

I did not notice that stamps.com billed me and only recently I contacted AMEX. Amex informed me that I might be on their trail and $15 was their monthly subscription and that I needed to contact them to cancel their subscription. 

I believed that this company is not upfront with me on the cost of the service. Would I be aware of the monthly subscription - I would not have signed up for the trial.  It is the responsibility of the company to inform through statement notification of any billing.