Insufficient postage surcharges, such as underpaid or short paid postage, refers to when a carrier determines that you have not paid enough to cover the postage on a package. For example, when generating shipping labels for your vendors, surcharges could occur if the vendor ships out items heavier than the weights entered by the label.
The most common mistakes that can result in surcharges include entering incorrect package weight, package dimensions, or destination address.
For more details on how surcharges are applied, we recommend contacting the support team for the carrier aggregator you are using. Shippo provides some useful details on this across common carriers within their support docs.