You are irreplaceable

I once worked with an agency as one of my clients, to deliver some writing projects for them.

Later on, I had more work pressure at my job, so I amicably let go of them.

Fortunately, they agreed. And also requested me to hand over my work to the next person who will take that on.

The new person was talented. They did some good work. I silently believed they will be able to fill in my shoes.

Alas, they wanted to charge for every small thing that they were asked for.

But this is not how you move ahead in life, my friend.
You have to learn to give more than you are asked for (without killing yourself).
You have to be okay to help if it is called for, once a blue moon.
You have to stop measuring your work for money sometimes, because it will yield you return for more times you could think of.

Of course, you should charge money if the additional services become a recurrence and not something that should be in the same bill.

However, to measure every action of yours by money is doing anything but increasing your worth. Wise people don’t fall for this trap.