Hello @EllenS41 ,
You can find a full drilldown of SMS pricing and tiers while signing up, though this is also screencapped and covered in our main article on SMS registration. The $10 tier is for 0-500 messages. This includes messages you directly send, as well as follow-up responses from prompts given by contacts.
For example, if you have 100 SMS contacts, and only plan to send 1-2 messages directly at most, then the 0-500 tier should cover you with room to spare. If you have 500 SMS contacts, then it'd be recommended to do a higher tier like 1k-2.5k messages.
You can find a wide range of helpful articles - instructions, videos, etc. - on SMS in our Knowledge Base, also available via the Help button in your account's top navigation bar.
Getting started with SMS (PDF download)
Registering your account for SMS
Creating SMS Campaigns
SMS marketing overview
SMS marketing best practices
SMS marketing while respecting quiet hours
Enabling SMS for landing page sign up forms
Importing SMS contacts list
... View more