It's basically that small packages, under a pound, go via Mail Innovations and the larger weight packages from 1 pound up, are designated Surepost. They both start at UPS and get handed off to USPS for the local delivery to the recipient.
My experience has been the same as others : Surepost gets to me timely and Mail Innovations takes so much longer. I track my packages and notice that it is always the post office end that stalls MI. UPS shows the handoff in the tracking progress and then the post office side takes days and days to move it through their system. Just because these are small packages like jewelry items shouldn't make a difference.
If you are a member of My UPS you can use the My UPS tracking page to request to keep a Surepost item in their own delivery stream and not hand off to USPS. If you are a paying member of My UPS it's free to request delivery to stay with UPS. Otherwise it's an extra $3.50! There is no such choice with Mail Innovations. Those small items always go from UPS to USPS and delivery is USPS only.
I think QVC does a disservice to its customers by using Mail Innovations for jewelry items or other small lightweight merchandise. The delivery interval is so annoying and unreliable that it makes me hesitant to buy that type of merchandise.