Hi,
Normally, after the payment is done, the status of the order changed from "created" to "confirmed" which indicates that the payment has been made for the order.
And then, HikaShop sends an "order status notification" email to the customer.
If the order doesn't change to "confirmed" or the email is not sent (meaning that you don't see it in the Customers>Emails history menu), you should check with the developer of the payment plugin.
If the email is in the Customers>Emails history menu but your customer doesn't receive it, you'll want to check with the support of your hosting regarding the sending of the email by their email server.