Estimate return shipping cost for a product by size and weight.
Carriers charge on billable weight, which is the greater of what the package weighs and what its volume implies. Dimensional weight divides the cubic inches by a divisor — 139 domestically for the major carriers — so a large light box is billed as if it were heavy. That is why returning a bulky item in its original oversized carton costs more than repacking it: the surcharge line shows exactly how much the empty space is costing.
Dimensional weight
Dimensional weight = length x width x height / 139
Cost
Cost = base handling fee + max(actual weight, dimensional weight) x rate per pound
Often, by repacking into the smallest box that protects the item. Reducing each dimension by two inches on an 18 by 14 by 10 carton cuts the dimensional weight by roughly a third.
It depends on the reason. Faulty or misdescribed goods are usually the retailer's cost by law in many jurisdictions; a change of mind is commonly yours, which is when this estimate matters.