Skt NURCaptha WP Plugin is a free software distributed under GNU GPL 2 License. You must not pay to make use of it. Nevertheless you may be kind enough to think about the hours the author spent on this code and feel inspired to give a token of gratitude to him.
The link below leads to a PayPal form where you can donate whatever amount you want. It doesn’t matter how much you give, the author will be grateful and prone to develop a few more pieces of free code for you.