Free shipping, the promotion offered most often by retailers last holiday season, is the email offer that's most guaranteed to be opened by customers this year.

According to online marketing firm eROI, a study of consumers found that the majority of consumers prefer free shipping over discounts, writes DM News (via MediaBuyerPlanner). Discounts, though, are also important to consumers, with the majority of respondents saying they would prefer to have a percentage discount than a dollar discount.

Also, consumers seem to want flexibility during shopping, with more preferring a 10 percent discount on any purchase than a 15 percent discount on a purchase over a certain dollar amount.