Encountered an issue with ZC 1.5.5b and OPC 1.0.7 installed (long time customer alerted me of this by phone - it pays to look after your customers.... others would just leave and business is lost).

OPC enabled as I had it enabled with all other versions of OPC. The cart is configured to allow free shipping for domestic orders over $150, has been like that for years and always worked fine.

The customer has goods over $150 in the cart thus gets the message

Free Shipping
Free shipping for orders over $150.00
Next the customer clicks the 'Confirm Order' button and promptly gets the error messages

Error Your available shipping options have changed. Please re-select your desired shipping method.
Error Your order's details have changed. Please review the current values and re-submit.
Ofc there are no shipping options and the customer runs into a dead end, can't complete the order.

I can reproduce this on a local clone of the site. But, when I remove an item from the cart and the OT goes below $150 then all is good, I can complete the order.

In the docs it says that OPC has been validated with

Shipping Methods

Per Item
Flat Rate
Store Pickup
Table Rate (and various clones)
United States Postal Service (usps)
United Parcel Service (ups)
FedEx
Product Delivery by Postcode (PDP)
but does not state free options.

I have a debug file for OPC and can email if required.

Cheers / Frank