In California a sweepstake is defined as a game of chance requiring no purchase or any other payment in order to participate in the game. The fact that you do not have to pay anything to be eligible for the prize is what makes a sweepstakes legal. So, if you send in an entry without a “proof of purchase,” it must be given the same chance of winning as an entry that includes a “proof of purchase.”

Many people mistakenly believe that purchasing the product and checking the “yes” box on the entry envelope will increase their odds of winning the prize. However, to be legal, each entry must be given the same chance of winning the prize, whether or not the entrant makes a purchase or checks the “yes” box. In fact, California law requires that a sweepstakes solicitation include a prominent statement in the official rules stating that no purchase isnecessary.
It is unlawful for a sweepstakes operator to sell or solicit for an information access service (e.g., a “900” or “976” number) in any manner related to sweepstakes. This would include requiring that sweepstakes contestants call an information-access service.

By law, sweepstakes entry materials cannot represent, taking into account the context in which the representation is made, that a person is a winner or has already won a prize, unless that person has, in fact, won a prize.