Which of the following is NOT an instrument being assessed by fixed duties?
A. Payment receipts
B. Cheque leaves
C. Guarantor forms
D. Proxy forms
E. Bills of exchange

E. Bills of exchange

Explanation:
Bills of exchange are typically assessed based on ad valorem duties rather than fixed duties, as they represent financial transactions with variable values. In contrast, payment receipts, cheque leaves, guarantor forms, and proxy forms are assessed by fixed duties.