Well, that Apache stuff looked promising, but didn't solve the issue. It seems to solve a slightly different problem (i.e. double parse special chars).
About the other idea, it is PayPal the one sending me those chars, not myself. And I am not worried just for PayPal, but for Prestashop in general. I mean, we all usually do UTF-8, but if someone comes with the other basic url encoding, cannot we support it?
I'm wondering... maybe Prestashop is validating URLs and does not accept this encoding? Is it possible? And, if it is, where do I look for that?
Thanks for the reply anyway.