Hello,
Good news: the order admin notification already gives you the customer's email, so no custom development is needed. It is there in two ways.
In the email body: the customer's email is output by the variable
, which is shown on the "Customer:" line of the default template. If your customized version of that email no longer shows it, just add it back where you want it. You can also use the generic tag
{VAR:customer.user_email}, which prints the customer's email wherever you place it in the body (the same dot notation works for other customer fields too, for example
). You edit the email under System > Emails, the "Order admin notification" email (HTML view).
As the Reply-To: HikaShop already sets the customer's address as the Reply-To of that admin notification, so you can simply click Reply in your mail client and your answer goes straight to the customer.
If the Reply-To does not seem to work, make sure you are on the latest 6.5.0 build. The Reply-To handling was recently hardened for Joomla 6 and PHP 8.