I had contacted Customer Service to ask them the same question so I'll tell you exactly what they told me.
Usually he never sends the Invoices, but if one of your Winned Prizes works more or encounters a malfunction, just contact them to Confirm them the prize and they will of course be charged the shipping costs.
A quick question to the webmaster about how to guarantee the Prizes won (wii, ps3, xbox, ds......)? Can we have an invoice for example? Thank you in advance for your answer.