Hi,
HikaShop checkout works, be it on the 3.5.0 or on the 3.5.1. It works for everyone else, and even on your website, I was not able to reproduce the problem and apparently other users of your website either. Maybe it's something related to your user account or your user session, or your browser, or the product you're testing with, etc, as I had hinted at in my previous message ?
The only errors resulting from the 3.5.0 were about the emails checks of Joomla which changed and we adapted the code of HikaShop with the 2.6.3 to take these changes into account.
We're happy to help but can't fix something which already works. And you didn't provide a way to reproduce the problem yet so I can't help. I'm indeed unable to investigate in these conditions.
You say it is definitely an issue with our software, but there is no proof that this is the case. Again, it works for every other website, and even on your website, when we tried, it worked.
I understand you're impatient but please understand that first we have timezone differences and we answer under 24 hours, not 2 hours.
I would recommend to review your update procedure (not only for HikaShop, but also for Joomla and other extensions of your website) if your website is a key piece of your business. In that case, you should first make a copy of the website, test the update on the copy and if all goes well, do a backup of the live website, update there and test again. That way, your website can keep being operational while you iron out the kinks, if any, and you're not pressured by the time.
Now we sure can refund you if you're not satisfied and don't want to continue using HikaShop. In that case, please follow this procedure:
www.hikashop.com/money-back-policy.html