How to integrate "other amount" (to donate) in PayPal payment

I did the paypal integration for the custom amount ($25...) but there is no way to integrate the last choice "other amount" which will allow our customer to input the donation not in the list of donation amount.

You can do that by passing the default and custom value to it.

To see how to do that, please take a look at the following guide: How to pass a calculation to a payment field

Of course, if you have any questions, do let us know about them and we would be happy to assist with them.