Hi,
I tried to reproduce the problem on your website with both my smartphone (samsung note 3), my Chrome browser HTC One emulator, and this emulator (
take.ms/qhZmw
) and in each case the popup was centered.
I don't have your phone and can't reproduce the problem with what I have on my end so it's impossible to analyze the CSS of the page. The popup is a mootools popup and the CSS comes from Joomla, so it's even not something to change in HikaShop.
So I'm afraid I won't be able to help with a precise answer.
All I can say is that if you want to fix the popup for your phone, you'll want to add custom CSS code, but you'll have to be careful not to break the display for other phones where it displays properly. If it works properly for most phones, you might not want to change that.