When you give the 3 free green bushes, why is there amount of 45 coins grayed out if they are free?

Only the first green bush is free. The following two cost 45 coins each. Once you give all 3, the button becomes gray.

Each person can only give so many bushes each day. The limit for green bushes is 3 per day. You can also send the other types of bushes if you run out of the 3 green bushes.

In total each day you can send 1 red, 2 blue, 3 green that cost coins and 10 special bushes that cost gems.

Occasionally there are more special ones that cost gems you could give on a day but typically the above amounts are what you are limited to.