Hi,
You said that they have the same issue with other browsers, so it's not a browser issue, and thus not a browser cache issue.
You said that it works for others, so it's not something general to the setup of HikaShop or the template.
Turning off the cache of the website might help if you have any. You might want to try that. Just clearing it won't help you know if it comes from the cache or not since you aren't able to reproduce the problem. If you turn it off and no one has the problem after the, then you'll know that it comes from the cache.
You said that you have limitations on the user accounts. What kind of limits ? How are these limits created ? Maybe it comes from these limits, which would explain why some people have the issue and not others...
Did you try to purchase with the user account of a user who had the issue ? Did you had the problem in that case ?
I would start with that. The first thing is to reproduce the issue. If we can't, it means that we don't understand the problem, and thus changing the settings randomly is just like trying to find a needle in a haystack.