The "w" switch is more accurate; however, the number switch, using a weight category, can be useful in special circumstances.
In both instances the cost is adjusted using your zip code to determine the "from" location and your customer's zip code to determine the "to" location. If a shipping destination is specified different from the billing location, then the shipping destination zip code is used. If your customer selects multiple shipping destinations, the first specified zip code is used to calculate the shipping cost.
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