If checkbox is checked, output variable * variable

so basically for my school assignment, i've been asked to create a application which allows user to create a tshirt, say how many they want, colour etc... And then get a total price, a price with gift wrapping (Check Box) added, which create grand total, and then if you're a business customer, you can get VAT off, of your purchase.

So for all of this, i am looking for a solution to

If chckBxGftWrp = checked, output Totalprice * GiftWrap to txtBxGrndTtl

Also if anyone were to be able to help me with making Business Customer checkbox (if checked) output Grand Total - VAT. So basically like

If chckBxBsnsCust = checked, output whatever is in txtBxGrndTtl - VAT(Variable) to txtBxBsnsCust

Totalprice and giftwrap are both assigned variables. I have radio buttons which allows customers to choose a size, where in this, ive used the code txtBxGrndTtl.Text = TotalPrice.ToString("c"); - therefore TotalPrice is a string.