That button is actually an image with a link.

Code:
<!-- ** BEGIN PAYPAL EXPRESS CHECKOUT ** -->
     <div id="PPECbutton" class="buttonRow">
          <a href="http://yourdomain.com/ipn_main_handler.php?type=ec" class="ui-link">
          <img src="https://www.paypalobjects.com/en_US/i/btn/btn_xpressCheckout.gif" alt="Click here to pay via PayPal Express Checkout"></a>
     </div>
<!-- ** END PAYPAL EXPRESS CHECKOUT ** -->
Look at the jQuery Mobile documentation for Linking without Ajax here. I states the following:
Links that point to other domains or that have rel="external", data-ajax="false" or target attributes will not be loaded with Ajax. Instead, these links will cause a full page refresh with no animated transition. Both attributes (rel="external" and data-ajax="false") have the same effect, but a different semantic meaning: rel="external" should be used when linking to another site or domain, while data-ajax="false" is useful for simply opting a page within your domain from being loaded via Ajax. Because of security restrictions, the framework always opts links to external domains out of the Ajax behavior.

All you need to do is find the file that create the code shown above. If it is a template file that can use the template override system, make a copy, added to the mobizencart template and modify it there. Otherwise you may want to make changes with an if condition. All you need is a data=ajax="false" in the href. Something like:

PHP Code:
if ($detect->isMobile()) {
href="http://yourdomain.com/ipn_main_handler.php?type=ec" class="ui-link" data-ajax="false"
} else {
href="http://yourdomain.com/ipn_main_handler.php?type=ec" class="ui-link"
} 
The above is an example and would not work but should give you a good idea.