Hi Mizou,
I don't know the application but the general IP rules apply to all.
AFAIK port 9050 isn't used on SME (or many other systems) so it shouldn't conflict with anything.
Your error:
When I try to connect (during checkout process), I get a message
(45)Failed to create a socket connection to Payment Client - Host:127.0.0.1 Port:9050
suggests that the software isn't running (and listening for connections on port 9050). To check, as root on the SME server run:
netstat -a | grep 9050
or if you've added an entry to /etc/services which identifies 9050 as a servicename the try:
netstat -a | grep servicename-you-created
If those commands don't show anything then it's probably not running (to be absolutely sure try "telnet localhost 9050" that should try to talk to the daemon directly).
Anyhow, it seems that the application and the web server all run locally (on SME) so you don't need to open ports - it won't help you and it'll invite connections from the 'net
Damian