Useful Tip for users of Sixt's US site sixt.com: try accessing the same site using a Canadian IP address. You may find your rate is cheaper than booking as normal from a US IP address. Your quote will still be priced in either US dollars or the local currency of the renting country.

Using this trick I was able to save about €50 off a rental in Spain.

The Canadian IP address didn't cause any of the other details to change, only the quoted rate. There is nothing in Sixt's T&Cs about not being allowed to access the site from a foreign IP address (and indeed there are many legitimate reasons to use one - being physically present in a foreign country for example!)