Getting it right
Choose reviews that answer the doubt rather than the ones with the best rating. If people hesitate over delivery time, pin the review that mentions fast delivery. If they worry about support, surface the one about support.
A badge near the button works where a full section would be intrusive — the rating is visible, the reviews are one click away, and nothing competes with the action.
How to tell whether it worked
Set up the comparison rather than assuming. Add reviews to one page and not to a comparable one, watch the completion rate for a fortnight, and you will have a number that is true for your business rather than for someone’s survey.
Be sceptical of published uplift figures, including anyone else’s. They vary by an order of magnitude and depend on category, price and how much doubt there was to begin with.