Used this app for a while. Occasionally it wouldn't allow customers to complete their purchase claiming that we didn't ship to the UK. Approx 2% of orders affected. Shopify adamant problem isn't with them. Tried to get Bytestand to look into the problem but after hearing nothing on the subject have given up. Voting with our feet and moving to another app which also offer inventory synchronization.
This is unfortunate as this shop owner was with us for over a year. We communicated many times over email with this customer, and explained Amazon will not return rates if they have no inventory. This is Amazon's way of saying they cannot ship something they do not have.