Not only are the $100 Holiday Rewards only good on future purchases, they expire 12/31/00. The following is from mercata.com:
Our Gift to You * Holiday Rewards
Purchase any of the following products from the Mercata store, and youâ€™ll not only get an amazing deal, youâ€™ll get a little something extra from Mercata... free store credit that you can use on any future purchase.
Each product earns a certain amount of Mercata$ store credit (specified below). When you purchase any of the selected products, weâ€™ll automatically credit your account with the appropriate amount of Mercata$. There is no limit to the amount of Mercata$ you can earn, and you can use them to receive up to 20 percent off any future order of a product placed between now and 12/31/00.