Hi,
While I can understand where you're going with this and do think you have a point, I think I'll side with NW on this one.
Consider this scenario...
You buy a sweater from a given Department Store Chain and used a "10% off any item today only" coupon (Yea ... I had to take my Mom all over town for the Day After Thanksgiving sales

). The sweater was to be a gift to your spouse. He/she opens it but it'sthe wrong size/color/whatever. You take it back but the store is out of them. They will be happy to refund the price you paid, but you've lost that 10% off any item coupon.
I can't think of any company that also refunds the "benefit of any coupons used". While you dont specifically state that you wanted the PD887 applied to the new ticket, in essense that's what you're asking for (a $75 difference). I can't imagine any company ever calculating a refund the way you ask (give credit for the full price even tho you didn't pay full price).
Just my 38 ...err... 2 cents worth
Steve