My advice, don't touch the billing, leave it alone, otherwise you maY get double billed and then you're gonna spend even more time sorting it out. I had a similar story, had a car rental which wasn't charged, the pre authorization was returned. I noticed the anomaly but didn't say anything because I know for a fact that preauths allow the company to take the money upto 12 months or so from the date of preauth. Come December, 3 months later sure enough it showed up and was payed in due course.
No need to complicate an already complicated process of billing systems. If you feel that bad perhaps order another ipad and return this one and hope the billing goes thru correctly the first time. Otherwise just leave it, they will bill you for it.