Onyx allows you to manually adjust the VAT on any particular row, this is extremely useful if you need to create a VAT only invoice.
To create a VAT only invoice just create an invoice in the normal way but when you add a part (or whatever it is that you are adding) you can now also adjust the VAT element.
Note that the Retail cost is £0.00 but we have changed the VAT amount to £100. Also note the small button to the right of the 'Retail Price' amount, this button will recalculate the VAT amount in case you have accidentally edited this figure.
And here we can see the item has been added to the invoice.