I would expect the taxable item list to change over time. This suggests the
Taxable value should be saved in the order details table. If the taxrate is
the same for all taxable items, you shouldn't need to save the rate in the
order details.
You can add the Taxable field to the productnumber combo box in the order
details. You can use the after update event of the combo box to set the
taxable value in the order details.
The product table has a check box for taxable items called "taxable"
The order table has a field called "TaxRate"
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