Don't worry about them changing the prices on your CC, upstanding stores like the major players [Staples, Office Depot etc] will never add to your credit card..
When you review the order and the coupon is shown as accepted, you're in !
Also, if you look at rebates they all say
"Not valid with any other offers or discounts"
I've NEVER had any rebate rejected due to using a coupon..
FYI '

'
Gluck
Doc