You can issue a receipt from the reservation details page.

<For registered users>
On the login page, select [Log in with your Kosoku Bus.com ID].
Log in with your registered ID (email address) and password.
Select [My Page].
Select [View / Change / Cancel Reservation].
Under [All Booking History], select the reservation for which you would like to issue a receipt.
Under [Total Amount], select [Issue Receipt].

<For guest bookings>
On the login page, select [Book as a Guest].
Enter your reservation number and phone number, then select [View Reservation].
Under [Total Amount], select [Issue Receipt].

■ My Page Login
https://www.kosokubus.com/_en/?page=guest_login