Finally got a customer from one of our sites to answer their phone and let us walk through the process with them.

The site has the latest WebPay from github and OPC without any changes other that the comment-outs suggested in WebPay.

The item is also Free Shipping

I have been able to test with VISA successfully every time.

Tried with a MC after this trial and got the error

This site is getting multiple sales daily and only an occasional ms-confg.

This customer got the usual ms-confg using DISCOVER.

She volunteered to use another card and I asked if she had AMEX.

She did and it worked flawlessly, first time.

So perhaps anyone who can test DISCOVER and MC to see if they get the same results.

Also, while it may be only specific to OPC, the item's free shipping throws the following error:

[27-Apr-2022 21:26:10 UTC] PHP Notice: Malformed value for session-based shipping module; customer will need to re-select: {"id":"","title":"FREE SHIPPING! (No Delivery Charge)","cost":0} in /includes/classes/order.php on line 323