I emailed one of my merchants www.classiccloseouts.com twice asking them if a customer follows a link from my site to theirs, and the customer opts for international shipping and are transferred to douwantit, would I get the commission.
I went through their demo and some of their FAQs. It looks like what happens is that people who live outside of the US can shop at US based stores that don't offer internationalshipping themselves. The purchases are actually made by DoUWantIt to merchant; they in turn sell and ship it from their centers here is the US to the international customer. The purchase it is actually processed through DoUWantIt's shopping cart and not the merchants. Since that appears to be the case, I don't see how you could get the commission since the merchant's receipt page is never reached. OTOH, you wouldn't have gotten a sale anyway, since the visitor is international and the merchant doesn't ship internationally.
It makes for a very interesting situation, since yes you did drive the buying customer to the site. But DoUWantIt's services are also necessary for the sale to be made. And technically, DoUWantIt becomes the customer to the merchant where there is a physical US address. I could find nothing on the site, like a merchant FAQ, that explained how the ordering is handled between DoUWantIt and the merchant.