One-Page Checkout [Support Thread]

[rlexyd]

First thank you for the plugin, it's a great idea :)
I am however having a problem with it.
I have a new install of Zen Cart v1.5.7c running responsive template on PHP 7.4.21 and only a handful of plugins installed.
I have two flat rate shipping modules installed (Regular & Express) after cloning the Flat rate module.
Whenever I go to checkout, I click on the radio dot of the "Standard Flat Rate" under shipping method but the screen quickly refreshes and the shipping method options disappear (view below snapshots) and can't confirm order as it will come up with error when I click on the confirm order button?!
I checked the error log and the below code is at the bottom of the log, can't really figure out the issue, I've had to turn off the one checkout module for now, the checkout process works fine in normal checkout mode, no such issues?!

(Please view snapshots below):
BEFORE pressing the radio button: https://i.imgur.com/D5tYjwq.png
AFTER (error) clicking on the radio button: https://i.imgur.com/Ogd7nIW.png
AFTER (error) clicking on the Confirm Order Button: https://i.imgur.com/htIzRAG.png

Wonder why it's doing this? I hope I can get some assistance.
Thank you.

2021-07-25 15:41:34 index: updateShipping, returning:{"status":"invalid","errorMessage":"Your available shipping options have changed. Please re-select your desired shipping method.","orderTotalHtml":"<div id=\"otsubtotal\">\r\n    <div class=\"totalBox larger forward\">$7.00<\/div>\r\n    <div class=\"lineTitle larger forward\">Sub-Total:<\/div>\r\n<\/div>\r\n<br class=\"clearBoth\" \/>\r\n<div id=\"ottotal\">\r\n    <div class=\"totalBox larger forward\">$7.00<\/div>\r\n    <div class=\"lineTitle larger forward\">Total:<\/div>\r\n<\/div>\r\n<br class=\"clearBoth\" \/>\r\n","shippingHtml":"","shippingMessage":"","paymentHtml":"<!--bof payment-method choices -->\n  <div id=\"checkoutPaymentMethod\" class=\"floatingBox forward clearRight\">\n    <fieldset>\n      <legend>Payment Method<\/legend>\n<div class=\"custom-control custom-radio\"><input type=\"hidden\" name=\"payment\" value=\"dirbankaus\" id=\"pmt-dirbankaus\" \/>      <label for=\"pmt-dirbankaus\" class=\"custom-control-label radioButtonLabel\">Bank Deposit\/Internet Transfer<div id=\"bankpayment\"><img src=\"images\/bankpayment.jpg\" alt=\"\" \/><\/a><\/div><\/label>\n      <\/div>\n      <br class=\"clearBoth\" \/>\n\n    <\/fieldset>\n  <\/div>\n  <div class=\"clearBoth\"><\/div>\n<!--eof payment-method choices -->\n","paymentHtmlAction":"update"}
Shipping not set