kindly advise

yes u can allow the customer to pay service tax there is no obligation in it

but your sales invoice should clearly says that customer is paying the service tax as ultimately u r paying the freight to the transporter

The service tax liability is on the person responsible for paying the freight. In this case you can say you are his agent.

The problems in future could be that the dept may say they do not know whether he has paid or not and therefore ideally he should pay the frewight himslef and pay the ST on the freight himself.

who is liable to pay service tax amount.? Person who pay the frieght.

In first case you are liable to pay service tax even though you reimburse the frieght amount from customer

If you are paying frieght, liability to pay service tax is yours. You can't transfer your liability to customer

The provision talks of consignee or consignor or his agent.

Therefore if paid as an agent the principal maybe liable.