Always charge sales tax

First, create a hidden field for your product and checkout pages. This hidden field's only purpose is to trigger the tax calculation every time. Just make something up. For example, it might be

<input type="hidden" name="T" value="Y">

Next, go to the taxes page in the control panel.

In the "Name" column (continuing with the above example) put the letter T. In the "Value" column, put the letter Y.

Then the "Fixed" column gets the digit 0 and the "Percent" column gets the digit 5.