Recycaball will recycle your tennis balls and pay you for the privilege. There are various formats in which they take used tennis balls off you and they have to be in certain condition, so not weather beaten or frayed etc. You can get a tennis ball collection bin in your club and receive 30p per ball.
They also support an array of charities from the proceeds too. It’s a win-win situation. For further information, look at the weblink: