I do not have a phone number to call and the client service that I contacted using the form on the website does not reply.
What to do? Anybody has experienced the same problem?
Do you mean you've sent to much BTC, or do you mean you think the service was overpriced?
If you've sent to much BTC => As soon as a transaction is confirmed, it's (allmost) impossible to "charge back", as soon as it has a couple confirmations, it's certainly completely impossible.
The only thing you can do is hope that the vendor refunds you, and maybe open a thread in the reputation section or scam accusation section if he refuses to do this.
If you think the service was overpriced, there's nothing you can do either i'm afraid