Tax is calculate only after refreshing the page. Please help!

[iKarev]

Here, http://bitdefender.techcommerce.co.il/index.php/bitdefender/total-security-2015.html if you click АГОРА, then blue button, you'll go to checkout page. There you'll see that ΦΠΑ (23%) is $0.00. If you refrech the page, it become some right price. Why only after refreshing? What I need to do to make the right price (not $0.00) without refreshing.

Sometimes it's right price at once, but it's because browser cash, it needs to clear.

Please help!