Just had a glance at your site. It looks like you are running Silverstripe CMS 2.3.0. This is more than a decade old and is no longer supported. It will also contain many security vulnerabilities.
Without having access the your site to debug the issue, there’s not much anyone on this forum will be able to do to help you. You might be able to hire a PHP developer to help you debug your issue. But I think you would be better off using that money to migrate your site to a more modern platform.
You could ask a web agency to redevelop/upgrade your website to Silverstripe 4 which would be relatively expansive. Given the nature of your business, it doesn’t look like you need a very complex website. I would suggest you switch to a SasS offering instead which is very affordable. Those are common options:
Sorry, I know this is probably not the answer you were looking for.