How to restrict reward points based on shipping method

I want to be able to use the reward system but only for customers who use a specific shipping method on each order.

This means, for every product they purchase, and checkout using free delivery, they are given reward points.

If they redeem the reward points they have, they must use free delivery as the delivery method.

All other delivery methods will not allow addition of rewards, or allow customer to redeem existing rewards.

Any idea how this can be done?

Half fixed this, have added the code to only allow rewards to be used if a certain shipping method is selected, otherwise an error will show asking the customer to modify the shipping method to use reward points.

Now I need to add the function to only give a customer rewards after completion if a certain shipping method is used.

Fixed and Done.