Use a value list, name it "bolean" with an 1 as unique value, when the checkbox is checked has the 1 value, otherwise is empty the closest value to 0.
As Juan says, no script needed. After you create the value list with only custom value of 1, attach a checkbox to the field. Format the checkbox width same px as height so the '1' disappears at its side and type your own label, such as AddTax To use this value in a calculation, it might look like this:
Case ( AddTax = 1 ; Amount * .03 ; Amount )
This says to that if the checkbox is 1, include tax otherwise list amount without tax.
Since AddTax checkbox is a Boolean, you can also write it as:
Case ( AddTax ; Amount * .03 ; Amount )