Each person can enter once per day 9/23-26 (postmark on that date). “Send a postcard with the participant’s first and last name, phone number, mailing address, email address, and the name of this Sweepstakes” to Sixt Rent a Car LLC Sixth for free Sweepstakes PO Box 460967 Fort Lauderdale, FL 33316.
I just wrote out an index card and stuck it in an envelope. Maybe that will work, maybe not.
There are 400 of these prizes for mail entries and it is reasonable to think chances will be good given the limited time of contest and limited word of mouth. (Prizes for prepaid rentals are separate.)
Terms are here,. The $300 certs can only be used on one rental. They cover any charges including taxes, fees and add-ons (take note of how that may impact credit card insurance if you rely on that; I would not use it unless the cert is applied after rental so I can have the option for it still to be charged to my credit card).
The It does not say limited to the US so I will assume it is valid at all participating locations in US, Austria, Belgium, France, Germany, Luxembourg, Netherlands, Spain, Switzerland, or the United Kingdom. see full list here.
This is a marginal case considering the effort and minor cost to enter versus a potentially modest benefit. If you have plans for an expensive rental in our of their locations, it may be worth a lark.