Hi,
I was thinking that you were using the third component JoomSEF.
I can read in a post:
The htaccess is set to Joomla-Standard (just Rewritebase is set to "/").
What about commenting again the RewriteBase line ?
I am not very friendly with that option, but it could maybe cause the issue.
@Zielinski: Do you have this option enabled too in the htaccess file ?
@bittrapper: This "Pay now" button, is that the button displayed on a page just before the redirection to PayPal ?
If it is, so it is not related with the previous issues. Which browser is using the customer ?