Yes, please see this thread regarding this vulnerability:
viewtopic.php?f=4&t=15232&p=41707#p41707Encoding your Google checkout URL produced by s2Member would make it more difficult, but it won't prevent this vulnerability entirely, because it would still be possible to tamper with the variables before being redirected to Google Checkout. So ... more difficult, yes. A long-term solution, no.
We are currently working to address this in a future release of s2Member.
viewtopic.php?f=4&t=15232&p=41707#p41707