If an Insta-Shopper doesn't complete a shop just right, you now have the option to request a revision. The revision request option is found at the bottom of the reviewer view of the shop:
Clicking Request Revision lets you send a message to the shopper, explaining the requested revision:
The shopper then has 24 hours to resubmit the shop. If the shop is not resubmitted within 24 hours, it is automatically rejected.