A Shipping source is where an item is shipped from.  Some customers have items shipped from several sources.  To be able to offer accurate shipping calculations it is important to use accurate Shipping sources.

You can setup your shipping sources here:
YourDomain.com > Modules > Ecommerce: Products (mX) > Shipping Sources
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Once you have setup your shipping sources, it is important to specify what products use each shipping source.  By default products will use the "Primary" shipping source.

You can specify a shipping source by editing your products and specifying it.

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If you need further assistance in setting up your Shipping sources please feel free to Contact Support