Yes, it's "legal" the website is just an Html code generator to accept fiat/crypto payments. I guess you wanted to mean "safe", in that case, I would say yes as well but there are more popular alternatives too. I know they have a Wordpress plugin in the directory and perhaps the code is even open source.
Edit:
I checked and the code is open source
https://github.com/moneybutton
its WP plugin https://wordpress.org/plugins/simple-money-button/
And here is an article mentioning it news.bitcoin.com/test-driving-the-money-button-simple-enough-for-a-9-year-old
(bitcoin.com link, forgive me
)