Doh! I completely missed that setting. Thanks for pointing it out – have enabled it now.
I have also enabled Zip code validation as an extra check.
Do you have any insight into how they managed to change the product price for the transaction. Is there anything I can do to prevent it or is this something that needs a change to the plugin?
(FYI “Security Token Check” is enabled – i.e. “disable” checkbox is unchecked)