We're looking for a some way to accumulate reviews/ratings that will roll into Google Shopping results.
That said, every system I've researched uses either (a) a super cheesy popup with sweepstakes or prizes or irrelevant shopping deals, etc., or (b) requires our customer to register on their site, or (c) all of the above.
Has anyone found a service (paid is fine by me) that is a little more considerate of the site being reviewed?
Google scrapes reviews from all the major review websites on the internet. It's up to your customers to leave the reviews, and for you to encourage them to leave a review on those sites. Try ReviewCentre, Trustpilot.