WHMCS Report File Edit

Hi, I am using the WHMCS billing system. I would like a freelancer to edit the sales tax liability report and add the following information I would like to add payment method to the report as this does not show. When the report is generated say for example 28/01/18 to 29/01/18 report will show all sales but i would like to show final total as invoice amount + (plus tax) - (minus) credits Below is example Say client invoice amount is £5.00 + tax 20% would make it £6.00 - minus any credit client had in account to take off invoice so say client has £3.00 credit £5.00 + tax 20% equals £6.00 - £3.00 credit equals £3.00 So the total would need to show £3.00 Hope this information helps