Okay, I placed another order and see the font size issue. You can turn it off now.
There are a couple of things that are wrong or odd. The usual font-size resetting to 62.5% is overridden, but no overriding declaration shows up in Firebug. Thus it is using the base browser font-size setting (100%). The difficulty here is that I can't see what is causing the override.

I get a CSS error message in the validator icon: Unknown pseudo-class or pseudo-element "a". Ruleset ignored due to bad selector. It then points to your stylesheet.css, line 16.
Your stylesheet has this
Code:
.myButton
a{color:#fff}.myButton:a:visited{color:#fff}.myButton:hover{background-color:#a81c72;color:#fff}.myButton:active{position:relative;top:1px;color:#fff}
which ought to be
.myButton a:visited{color:#fff}
This doesn't seem like it should be related to the problem.

There is another CSS error: An unbalanced tree was written using document.write() ... and refers to "line 25".
This indicates a javascript error, which I can't help find or fix. The likely source would be the affiliate javascript.
Line 25 in your page's view source is

<script type="text/javascript" src="min/?f=/shop/includes/templates/saffire2/jscript/jscript_imagehover.js&amp;1327610682"></script>

which wouldn't apply here and not on other pages.