Packaging comparisons become misleading when they count empty objects rather than equivalent deliveries. This methods note develops a functional-unit procedure for a small food service, drawing on the GHG Protocol Product Standard, the European Commission’s Environmental Footprint recommendation, and the Environment Agency’s original carrier-bag assessment. A hypothetical returnable-container case distinguishes completed meals, realised reuse, delayed returns, washing, replacement lids, collection journeys, and food loss. The analysis explains why consistent boundaries and evidence of displacement matter before environmental rankings are attempted. It also keeps accessibility and food safety outside any simplistic carbon score. No packaging footprints or savings are estimated. The proposed decision record identifies which operational observations and supplier information would be needed to support a defensible local comparison.