'Drawee must agree to bill of exchange by signing on face of instrument'

You can pay duty 'under protest' by giving a protest letter and taking acknowledgement

Q. The drawee has not accepted the bill of exchange by signing on the face of the instrument but has given a separate letter of acceptance. Is this in order? 

As per Section 7 of the Negotiable instruments Act, 1981, “after the drawee of a bill has signed his assent upon the bill, or, if there are more parts thereof than one, upon one of such parts, and delivered the same, or given notice of such signing to the holder or to some person on his behalf, he is called the ‘acceptor’”. Thus, the drawee is necessarily required to sign