Essentially your package is charged based on size and weight, but there’s a bit more to it than that. Generally, the heavier it is the more it will cost. The same is true for size. It’s figured by size or dimensional weight (DIM). Dimensional weight is when your package shipping price is based on size rather than actual weight. Dimensional weight kicks in when your package takes up more space even though it may not weigh that much. The carriers will charge based on what is greater the actual scale weight or the dimensional rate. Here are some tips for getting the best prices on your shipping:
If you have lighter items, try to get them in the smallest package possible so they take up less space in transport
Measure your package at the longest points and remember to round each measurement to the nearest whole number
If you don’t have a shipping scale you can use a bathroom scale to get a weight
We typically recommend rounding up to the nearest whole number to avoid carrier adjustment fees after the fact. If you really underestimate your weight the carriers could return your package to you rather than delivering it
You can assume that lighter items in larger boxes will cost more because they take up more space to transport, likewise, smaller items that weigh more will cost more because they weigh down the mode of transportation. If you wanted to delve into the calculations to find your DIM for UPS and FedEx multiply the length by the width by height and divide by 139. This calculation will give you your DIM weight. The USPS multiplier is 194 and applies to Priority Mail larger than a cubic foot going to zones 5-9. The greater of this calculation or the actual scale-based weight will determine the shipping cost. Here’s a link a great video J.M Field Marketing and Fulfillment put together as well. It can also be found on Shipthrifty for future reference.