Look on the Offers For You tab when logged in to the American Express web site for $20 back when you spend $100 or more at the HP Home & Home Office online store. Save the offer and then make your purchase. The $20 back offer expires 11/22/2013.
Combined with the OPEN savings, you should get a statement credit based on the after-tax amount:
If you spend $100, the combined statement credits are 25%.
If you spend $500, the combined statement credits are 9%.
If you spend $1000, the combined statement credits are 7%.
If you spend $3000, the combined statement credits are 9%.