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Rounding variances were being caused in some instances when POS form is was closed and new sales order is was processed
Solution
The POS plugin was calculating the line total independently of the sales order, because the POS will ignore back orders and assume the whole ordered quantity is being billed. It was incorrectly working out line total as (inc price * ordered quantity) - but in reality the tax is calculated on the extended total, not the inc price * qty (except in the case of items which are Sell Price Inc Tax Items).
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