I agree with feline fanatic's advice regarding contacting the quilt shop and asking what problems they see with the quilt.
How did you deal with starts and stops? Did you backtrack, or did you knot and bury the threads? That might not be a good idea with knits, since the knots might not be large enough to stay buried. Did you use a larger than usual seam allowance for the knits, so that they won't ravel? Are they the type of knits (e.g. T-shirts) that should have an interfacing? Is the quilting sufficiently dense? Is the quilting stitch length too large? How about the stitching in the piecing? Is the quilting tension good? (It looks okay in the pictures, but are we seeing both sides of the quilt? Tension can look great on top, but terrible on the back.) Those are some of the things that would affect the structural integrity of the quilt, and we can't really judge any of those issues from the pictures.
I want to say this nicely, so please don't take offense. Your comment about how much time you spent on the quilt, compared to how little you charged, is worrisome. It implies that spending more time on something excuses a poor product, and I don't think that's how you feel. Customers want to get a bargain, but they expect to receive a good product, no matter how much they pay.