I understand your problem and you have pointed out why it is important to evaluate all the paths along a secure transaction.
The reason GET is not secure is because the URL itself is stored in the server log
To be clear, GET over HTTPs IS secure. The problem along your path isn't with the communication mechanism, but the insecure part is with how your HTTPs server decides to store the URLs in its logs. There isn't a way to send POST variables with Switchvox so I would look into how you can change the log levels on your HTTPs server to be more secure. This may be a quick google search, or if you post the web server you are using someone on this board might have some insight how to do this.
I have entered your feature request to have the preference to send POST variables with the Switchvox product.