Since you are the ones currently in control of the site, you need to assign them the administrator role of the WP plugin site under the WP “general settings”, change it to administrator, by providing them the login credentials they will have full access there are no licenses on our end).
If they own the SLP products by virtue of you having purchased it for just their site and having they pay for it, give them the account info you used to create the site so they can log on with the SLP account. Provide them with the log in account info after you have changed it to what they want for the user name. You do not need to have User Managed add-on.
(Assume you have developed for other clients so this should be no different as far as how to deliver the product, follow your normal business practices)
Thanks and good luck in your future endeavors!