You can get a shortcode in three days - $500 for a random 5-digit, $1,000 for a vanity 5 or 6-digit. The biggest gating item is the approval process with the carriers. That will take 6-8 weeks.

If you're getting a prototype working, textmarks might be the way to go. You'll have to prefix your SMS command set with a keyword. Longer term you want your customers/members to identify with your own shortcode. Imagine them getting messages from two services on 41411