How do I apply discounts or add tax rates to the invoices?

To apply discounts to invoices in BUSY you can follow the steps 
if you want to apply discount item wise then follow the steps
Step1 go to administration menu and click on configuration
and click on voucher series configuration 
Step2 select voucher and its series and click on voucher configuration 
Step3 enable item wise discount and select the discount category 
you can select discount percentage wise or absolute amount wise on price list price 
Step4 then open voucher and after entering item quantity a box will appear where you 
can place you discount 
If you want to apply discount on voucher then follow the steps 
Step1 on voucher module bill sundry option select discount bill sundry 
To apply tax rates to the invoice follow the steps 
Step1 you need to ensure that tax category mentioned properly in item master
if it does not mentioned then open item master in modify mode by clicking alter and M together
from voucher after selecting Item 
Step2 put the tax rate in item tax category and save 
Step3 then in voucher module select sale or purchase type if the purchase type in tax inclusive 
or item wise then just enter after putting all the details of item like quantity rate amount and if you 
are selecting sale purchase type as multirate or any other rates then press F4 key mentioned
on above bill sundry box