How to investigate the issue
Parcel carriers can charge based on volume as well as scale weight. Measure the packed parcel, not the unpackaged product. Multiply length, width and height in centimeters and divide by your carrier’s volumetric divisor to estimate volumetric kilograms. For a 40 × 30 × 25 cm package and divisor 5,000, the volume weight is 6 kg. When its actual weight is 5 kg, this common rule produces a 6 kg billable-weight estimate before rounding or service-specific conditions. Different shipping providers and services may have different divisors, size limits, rounding and surcharges. Enter the real divisor from your carrier rate card and compare expected checkout postage with your full fulfillment cost. Reducing excess packaging volume can save more than cutting product weight in some scenarios.
Relevant free diagnostic tools
Official reference
Google Merchant Center product data specification. Check current policies and your account for definitive eligibility details.