Hi sorry it's taken so long to get back, my DotNetNuke foum notifications never arrive.
The answer has nothing to do with DNN. You create a Text/HTML module with this code
<div align="center" class="normal" style="width: 650px;"><iframe width="650" height="700" frameborder="0" border="0" src="/portals/0/googleCheckout.aspx">
</iframe></div>
and then that iframe contains your REAL Google Checkout code.
I just changed to a different solution today because this solution does not work nicely with Google Analytics, but with Google Checkout you'll be fine with an Iframe like this one. I've tested it on different browsers, too, and they're all happy.