With orders of $25 or more, Amazon offers Prime members the Kraft Mayonnaise 5-oz. Jar 12-Pack for $6.23. Clip the 20% off coupon on the product page to drop the price to $4.98 at final checkout. With free shipping, that's about $1 less than you'd pay for two 30-oz. jars in local stores. (Non-members can spend $35 or more to qualify for free shipping.)
Note that this item is part of Amazon's add-on program and can only be purchased with an order of $25 or more.