If an end customer is looking for a copy of their invoice or their payment details, they should be referred to the payment method they used at checkout. The invoice and payment details are held by the payment provider, so this is always where the customer will find them.
End customers do not have access to the Kustom Portal — the Portal is only for you as the merchant. It is not possible to give a customer a login to retrieve their own invoice or payment information.
If you are unsure which payment method the customer used, you can look up the order in the Kustom Portal to confirm it, and then point the customer to the right provider.