Are you using IE, or FF?
I can tell you not to be sketched out, just accept the certificate and try it.
".sslpowered.com" is the secure part of the website. The actual website where you add stuff to your cart, is not secure. The part where you checkout is secure. They are hosted on 2 servers. So certificates see them as 2 websites even though they are the same. That is about the best explanation I can give...
If you use the PayPal Express checkout, it will all be done through PayPal directly if you are more comfortable that way. If you use the regular PayPal checkout, it is intercepted by our website, we process it add it to the queue, and then forward to PayPal for payment. If you use Express checkout, it will take longer for a confirmation because it goes from the site to PayPal and we have to manually verify payment and add your order to the website.