I guess that this point you don't want to create a new landing page via launchrock.com because you probably have a web presence already.

Have a look at existing email list providers. They take care of double opt-in, allow you to download the email addresses and (if you wish) you can email customers fast. Better than reinventing everything again. Sounds like your core business is an app, not list management.

Example: http://help.campaignmonitor.com/topic.aspx?t=13. On the pricing page look at 'per campaign'. It's really cheap. (I'm not affiliated, there are probably other providers with similar offers)