For just bulk messages, with no payments, don't pay for a service.
You can send texts as email. Just find the email extensions for the various mobile services.
For example, for sprint, just send messages to
[email protected]
Any ten digit number on sprint sent to @messaging.sprintpcs.com goes to the user.
So, collect the numbers, and the carrier... enter it into a database... have your script add the address on the ten digits, and just treat the blasts as if they are email blasts.