The form of your payment shouldn't matter overly to the merchant when it comes to giving you a discount to earn your business. If they don't employ a check guaranteeing service then they'll incur a slightly higher transaction cost when accepting a charge, and that might affect the bottom line discount they're willing to negotiate a little bit.
Zaikesman
About 2% to 3%. That is what the credit card company charges the merchant when a credit card is used for payment.