Regarding the coupon, try telling them that you received it in a special email from Staples.
When I ordered my drive, I had no problems with using the coupon (I actually naively told the rep it came from bigbigsavings! But when my drive didn't show up Friday like it was supposed to, and I called to complain I got a CS rep that told me I shouldn't have been allowed to use the coupon in the first place. She told me something about an email they send out with the coupon codes, so that's why I'm assuming that you should tell them email.
Regarding the price - as others have mentioned - if you just PM with Office Depot (which is selling the retail version for 159.99), buy the ATA adapter for $49.99 at the same time, your total price will be roughly $210. Take off $40 for the $40 off $200 Staples coupon + free shipping and the $50 MIR from Maxtor and your total price is $120 + tax for the drive and adapter.