I'd enter the full purchase price of the things you're purchasing, then add a negative line item to the purchase for the discount amount - rather like you were entering a discount on a sale (only backwards) - using either the same account used for the purchase or an account used for purchase discounts.
if i use the same account (an office expense) the expense amount does not reflect true costs of operation (to me). I may try the purchase discounts approach but that treats the reward like income. Do I understand that correctly?
Let me step in and assist you with your concern today, @lme20.
As what brconsultingid2 suggested, enter the full amount of the purchase then add a reward item in the second line with a negative amount. If the account that is associated with your purchase is income, it will also treat as income in your QuickBooks account. To ensure that the posting amounts on your account in a financial report is correct, I recommend consulting an accountant.