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Shipping charges would be determined by the merchant, and I don't think any third-party service could accurately compute the correct amount that each merchant would charge for an order. (I'd love to be wrong on this, though.)
Sales tax can also be tricky, because computation of sales tax may actually be affected by the shipping charges -- in general, handling fees and flat-rate shipping charges are supposed to be included in the taxable base in some states (but probably not in others), while "actual shipping charges" are not subject to sales tax (in some states, perhaps they are in others). In addition, of course, you'd need to know the proper classification for sales tax for a particular product in a particular state (in some states, food is exempt from sales tax, except certain types of food [soda, snack foods may be subject to sales tax in some states).
The only "likely accurate" way to know what a merchant will charge would be to query the merchant's server (ideally the merchant would provide an API for affiliates for this purpose, but I'm not aware of any merchants offering this). Some datafeeds include a column for shipping charges, but I think a majority of merchants leave this column blank, and when the data is present it's definitely not always reliable.
Good luck on this.
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