Adding tax after product have been added

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Hi guys,

 

Once I started adding product to my store a while ago, I had problems with the numbers (extra zero's etc) when tax was enabled, so I simply put the whole price including tax, and then set there to be no additional tax.

 

However, now that I view invoices etc, obviously, the total excluding tax and total including tax are the same, which will be a hassle for customers who wish to claim deductions etc. So, is there a way that I can add the 10% GST Tax and take 10% off the exc. tax price, without doing it manually which would take forever?

 

Thanks alot,

Josh.