We use a URL formulated in-house called the OrganicURL. There isn't too much we can share in detail as this is what helps set us apart from other promotional sites, but overall it gives the appearance that an organic search for your targeted keyword was performed by the buyer which is followed by a purchase. The steps the customer takes is what gives you that keyword boost that you are looking for and we have simplified this process using the URL.
We do allow you to use any URL of your choice, but this Organic URL is an extremely powerful ranking tool. We do over 250 launches/day and have done over 30,000 in the past using this URL, and we haven't seen any kickback from it. You'll surely see much better results than with using a canonical URL. That being said, we want you to be comfortable so you have the choice.