I'd say just phone them up and explain it to them. It's not that unusual to have lost or thrown the receipt away if it really has been two years!

Trouble with shipping companies is normal
DHL are normally quite reliable but I've had a few packages which seem to have been routed via a war zone and been kicked around a fair bit as well. I've had no trouble with Fedex but they did lose a package of mine for a few weeks but it turned out that I hadn't filled in the commerical invoice properly and they can't track stuff when it's in customs. UPS seem to enjoy telling me that I don't live at my address anymore and send stuff back...

- Trevor