How can I activate a javascript button?

Hello, any help will be greatly appreciated.

I have a sale form I use for work.  Some products, we have to charge 7.5% sales tax, but not on other products.  I am hoping to be able to create something like a checkbox next to each sale item I can click so that 7.5% sales tax is added to that line item.  The way I currently have the form set-up is that I multiply the subtotal box * 0.075 and that gives me the sales tax total (for the entire sale).  Thank you in advance.