Ok

Discount coupon is set at $50

But when it is applied it gives $54.55 off

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My guess is that somehow tax is getting into this, as in Aussie the GST rate is 10%

so $50 Tax inclusive would be $45.45 + 4.55 tax and somehow or other this is getting the tax added on top of it again

BUT if you look here

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You will see that tax shouldn't be being applied.

Basically all we want is for a $50 discount voucher to mean $50 off, where are we going wrong?