Shoulda coulda
First off, never buy into these "deals". You live you learn.
You should have started tracking from day one... You admit that. As soon as you would have noticed the error from Sleep Number... You would have been able to call it to their attention then and there and be done. However, you didn't track it.
I'm not trying to be rude but as a business owner I check my business and personal bank accounts and credit card daily for any discrepancy.
Now... Previous experience years ago as a part time call service rep for a well known company who offered " free" life insurance on the original sign up for the credit card for 3 months. On my job, people called in because oh wow they just realized they'd been paying $6.95 a month on the credit card for life insurance and wondered why it was there. I explained the original deal where they were supposed to call in and cancel at 3 months if not interested.
Most ppl never saw the charge until woefully later. Depending on the attitude of the caller I could give an automatic 3 month reimbursement. If they pursued, I could go 6 months and I based it on the attitude I got. If someone was bitching me out... I gave 3 months then sent them higher up.
Bottom line it was their responsibility to look at the card. There was one person I recall who was so angry and said they had paid this charge over 6 months. I checked and said " sir you have paid this over 2 years". The max that person got was 1 year.
I think if your are very sweet, direct, to the point and state your case... You might get a little more out if them but bottom line part if the responsibility falls on you to check your statements regularly.
Good luck. Btw... We are thinking of a sleep number bed. Is is worth it? Do you love it? We don't do deals like you explained so I'm sure I get a higher price because I don't play their game.