Hi,
PHP is a script language: something (like Apache) calls the script (in that case Joomla) to generate a page, and the script returns back the page to be displayed and then exits.
So when HikaShop sends an email to your email server, it has to send it before PHP exits and thus it cannot delay it.
To have HikaShop being able to delay emails would require storing the email in a "queue" table and have a cron to check that queue and send the emails in it after the delay you want. While that would be possible with a custom plugin you could develop, there is no such functionality in HikaShop.
Now, HikaShop only transfers the emails to your email server and it's your email server which sends them to the receiver.
Email servers do have settings to delay emails. For example, for postfix:
pintant.cat/2010/04/14/how-to-delay-the-mails-postfix/
But we can't help with that. You need to see that with your server administrator and/or hosting company.