Reviews are the cheapest marketing there is and almost nobody asks properly. The moment to ask is the moment the customer says thank you — not a week later by email, when the feeling has worn off and your message is one of ninety.
Have the link saved on your phone so you can hand it over there and then. Ask for one specific thing: what they were worried about beforehand and whether it happened. That produces a review that answers the next customer’s actual question, instead of another “great service”.