Which email are you referring to? Is this an email that by default uses the shipping cost meta tag, or one that you added to a notice that didn't have it initially?
Note that if this is for a WBN, where it's telling users to complete the checkout process - the cost of shipping wouldn't yet be finalized and as such wouldn't show up in the meta tag for it. If however your using this tag in an email that is sent after checkout is completed, where the shipping cost would have already have been finalized - then it should be populating.
