Need to pay tax/service tax if turnover is below 20l?

GST Tax will be paid in all the cases.......

If you are  registered with GST........ You will collect from customer and pay to government.....

If you do not wish to get registered (as basic limit permits for that), in that case your liability for tax ceases, but the registered customer will raise self invoice and pay the same amount to government.......

So, What If my customer will not pay tax? I will be held responsible?

I repeat: When you are not liable to register under GST for any of the causes under the act.........   your liability to collect tax and pay it to goverment doesn't exist.

If the liability to pay tax under RCM is over customer....... you will not be held responsible for his non-compliance......