I am just posting the current working solution for the error: "Your order's details have changed. Please review the current values and re-submit."

In my case, it only happened when a customer selected the moneyorder payment type and immediately trying to submit your order. Because the moneyorder payment type didn't refresh the page, and because no further information was changed, the checkout form was throwing that error for some reason.

So to to fix this, under the OPC Configuration menu, I added the 'moneyorder' option to the "Payment Methods Requiring Confirmation". So now it looks like this: eway_rapid,stripepay,gps,moneyorder

Now when a customer chooses the moneyorder payment option, it will take them to a new page to confirm their order, and this eliminates that message from popping up.