Banker’ includes any person acting as a banker and any post office savings bank. In terms of this section, a post office savings bank is a banker under the NI Act

Negotiable Instruments Act, 1881 - Section 3 - ‘Banker’ includes any person acting as a banker and any post office savings bank. In terms of this section, a post office savings bank is a banker under the NI Act. (Para 11) 

Pradeep Kumar v. Post Master General, 

2022 LiveLaw (SC) 139 :

 (2022) 6 SCC 351