I followed the user actions that you specified in your post and I see nothing at all wrong. The McAfee SiteAdvisor symbol remains grey even on the checkout page, and that's because your site is not yet tested.
You didn't say whether the customers reporting a problem were already logged in at the time they arrived at the Checkout page. Possibly the page - which has a secure connection using https - may have some content which is delivered over a non-secure connection? That would not cause a Red SiteAdvisor icon to show, but would (in some browsers at least) show a warning somewhere on the addres bar.
Your site is clean according to Sucuri, Google Safe Browsing, Norton Safe Web, &c.
Message was edited by: Hayton on 16/03/12 01:48:47 GMT
first of all, your website is green now: http://www.siteadvisor.com/sites/wickedtronics.com/
We also looked into your issue indicating a warning at the shopping cart, but we are not able to reproduce this behaviour.
Please follow up with us, if you are still seeing/hearing about this issue or if the green rating has solved this.
Just an update,
I contacted McAfee and they replied with the following:
"We experienced a temporary issue yesterday.
This issue affected www.wickedtronics.com, causing the temporary blocking of this site.
The issue was addressed and resolved within hours. The SiteAdvisor rating of this site is Green."
Thank you to everyone that replied.