How to apply custom tax rate after a condition before checkout

[rpandeyp]

Hello Team,

 

 

I just need to know how can i add a option for customer on cart summry page, if customer have "Form C" then all cart products tax rate revised with 2% instead of added 12.50%.

 

Please help me, where can i add custome tax rate for all added products in cart.

 

Thanks.