I\'m building a shopping site which uses a service which allows credit card payments (tranzilla), the service is requiring an IP or IP mask from which it will allow requests
You can also use Apigee to wrap the API in question - http://app.apigee.com/, and make your API call through Apigee instead of directly to the API in question. Then enter the IP that the Apigee call comes from, and you're set.
Yes, you can use a hosted VPN service to maintain a static IP Address in which to contact tranzilla. A proxy server might also work, however, using a public proxy server would be a bad idea.
There is now a new Heroku add-on called Proximo that gives you a static outbound IP address to connect to services with IP whitelists! At this time it's in private beta, and there is no pricing info available yet, but it seems like a promising solution: https://devcenter.heroku.com/articles/proximo