<p>There are a whole raft of questions regarding payment processors for <b>web</b> applications but I haven't seen one yet for desktop applications. What are your experiences?</p>

<p>Perhaps to put this into a little context, the requirements:</p>

<ol>
<li>Should be customisable</li>
<li>Should be available in Australia at least, if not world wide (less critical).</li>
<li>No storage of credit card details</li>
<li>Recurrent micro payments of varying amounts each time</li>
<li>1-click payment. It's ok for the user to be registered with the payment processor provider.</li>
</ol>

Times may be a changing, but I don't think that you can get all of the above.
I do have some experience a couple years back with credit cards in North America.  At the time the company I worked for needed recurring billing for a different amount each month (much like your requirement). At the time, none of the providers I looked into allowed for recurring payments where the amount varied. This may have changed, but I do not know of one.
I suspect your options are either to store credit cards and process your own transactions (not too hard but a path you have to walk carefully) or use recurring billing with a constant amount.