Hi,
In that case, I would recommend modifying that button to use HikaShop's contact form page (you can create a menu item of the type HikaShop product contact form via the joomla menu manager to ge tthe link to it).
That way, it will use the "contact request" email of HikaShop when the contact form is submitted, and you'll be able to customize it via the System>Emails menu of HikaShop.
Allowing customers to modify orders is not possible.
It would be problematic if the customer removes a product or add one while the payment has already been done.
Now, having such contact button on the order details page, without the need for a customization to add it, is something we could look into and which looks like an interesting idea.