It is always free to receive SMS. You’ll want to make sure data roaming and cellular data is disabled. You could even remove the “international data roaming” add-on in self-serve, which will mean you can only send/receive calls and SMS. Then you don’t have to worry about errant or background data use costing you 16 dollars a day. And you can still receive texts as you wish. Another option, if you’re going to change your plan anyways, is to port your number over to a voip service. You can pick any one that looks good...I personally use Fongo. But there’s many others. They’re free or very low cost, and allow you to make and receive calls and texts with a data connection. 

Receiving two-factor authentication (2FA) messages from financial institutions and online accounts that require this type of authentication may prove unreliable with some VoIP services. In my experience with Fongo and 2FA, it’s hit or miss. Sometimes it works, and other times it does not.

Your best option is to get the $15 starter plan and follow Goran’s recommendations on disabling data, data roaming and removing the international data add-on.