You enter in the cupon codes before you're actually charged anything, so you can always cancel the order if it doesn't work.
Dell is just kind of screwy. Each department can run its own promotions, and set prices differently. A lot of times the small business department has lower prices than any other for the exact same stuff. So it's just a giant game of trial and error to try and find the best price, if cupon codes will work, etc.
Hey I am from CAnada and am wondering if these coupons work up here in the great white north