I need help understanding secured PayPal Express Checkout via my Webshop.

So I basically try to make a lizens system. At the web shop you can add an Server IP and buy my stuff for it. Now I don't know what to do about checking out. I want to use Express Checkout via PayPal but the JS API provided by PayPal seems pretty insecure.

Now should I use the Official PayPal API or should I use an PHP API found on the Internet?

And other things that could help my Webshop are welcome to!