Post-shipment amendment under Section 149 requires existing documentary evidence and departmental scrutiny before shipping bill conversion. Amendment before LEO is processed by the Inspector and approved by the Appraising Officer/Superintendent after verification of documents under statutory amendment conditions; suspected evasion is referred to investigative officers. Post-shipment amendments require a documented application with supporting evidence, assignment of a file number, scrutiny by the Appraising Officer/Superintendent, issuance of a deficiency memo if needed, and final approval by the Assistant/Deputy Commissioner when documentary-evidence requirements are satisfied.