The SPAM Act applies to all message services, whether they are standard or premium in terms of cost. Further info can be found here.
There are three key points to follow the be compliant with the Act:
Only send commercial SMS with the addressee’s consent - either express or inferred consent (the are defined in the link above)
Include clear and accurate information about the person or business that is responsible for sending the commercial electronic message (that is, you or your business).
Ensure that a functional unsubscribe facility is included in all your commercial messages and deal with unsubscribe requests promptly.
To help you manage the unsubscription process required by the SPAM Act, a default Unsubscribe Rule is created in every account upon registration. You can remove this rule or edit it (and add more keywords) as you see fit.
If your campaign cannot receive replies (for example, because you're using a sender name), then you will need to have some sort of other way for them to unsubscribe. This could be something as simple as having a phone number for the addressee to call to opt-out.