Start with the guide that matches what you are sending. Each one names the service to buy, the packaging that keeps the parcel under the next price break, and the failure mode that costs the most for that item, whether that is crushing, creasing, breakage or a dimensional-weight charge.
If you already know what the packed box weighs, go to the cost-by-weight guides instead. If you are choosing between two carriers rather than two boxes, go to the carrier comparisons.