I think something like this will do it
<rewrite>
<rules>
<rule name="Rewrite to article.aspx">
<match url="^/([_0-9a-z-]+)" />
<action type="Rewrite" url="yourshoppage.aspx?shopname={R:1}" />
</rule>
</rules>
</rewrite>
please check if you have other rules because the order might be affecting that. Also check your regex etc.
hope it helps