Help with website maintenance.
Once the website is live, it can demand some adjustments on the spot or later on as your client starts receiving feedback from visitors and customers. It happens all the time no matter how thoroughly you've tested it before going live. So it's common courtesy to help your client out with those in the first month or two from the moment the website has been launched.
Whether you should view this work as an extra one and charge for it additionally depends on how much time those adjustments would take. If it's just 10 minutes or so, you should probably make it on the house. If it takes hours, of course, ask them to pay for the extra work.
The important thing here is access to the website's admin panel. Once the website is done, it belongs to your client. But in order to be able to quickly assist them with any problems that may occur it's better for you to have their login information. So just politely ask for permission to keep it. Some clients will not want to share passwords—in this case they can grant you with full or limited access using Tilda Collaborators.