Updating PHP checkout page

Hopefully I will manage to explain my problem and you will have a solution for it :). I have a checkout page, coded in PHP (by some other coder, not available anymore). On the page I have one table containing shopping cart items and one submission form, to collect all information from the customer. When the page is loaded, the table is filled with data from the temporary data table through SQL query (collected on the previous page, shopping cart page), except for the field about the amount of the sales tax, which is not yet known because I don't know anything about the customer. So I have a shopping cart table and a blank form below that, ready to be filled with customer's information. Now, my question is: How can I calculate and fill the sales tax field in the shopping cart table, if the customer chooses his state from the drop down list in the form? I must say here that the sales tax is only applicable for residents of just one state, Vermont, because the company is based in Vermont. For instance, if the customer chooses Vermont as his state, how this can be translated into sales tax amount, added into the field in the shopping cart table and recalculated the total amount? Should the page be reloaded, with changed value of some variable for the sales tax? Should the JavaScript be used? All information entered in the form should be kept after the selection of the state and calculation is made (I assume that the page should be reloaded/refreshed), so the user shouldn't have to re-enter them again. After this recalculation, the customer hits the "Make Purchase" button and the form is submitted and the receipt for the purchase is sent. I'm not very familiar with PHP or JavaScript, so any help is greatly appreciated. Thanks, Robert