The reality is that there is generally a lot of leeway in "fair" as far as contracts go. You can definitely sign an intentionally adversarial contract and it will usually hold up. I have only known courts to nullify contracts when they specifically violate a given law; for instance, a contract where one side can kill the other side in event of breach would not be valid because it runs afoul of murder/assault laws. However, a contract where an exorbitant fee is levied upon one side in event of breach would generally be held as long as all parties were deemed capable of understanding the contract and endorsing it by their own free will (i.e., not under duress) when it was signed.

IANAL, this whole post may be and probably is wrong. Please correct me.

This is a complex issue, but there is no general fairness requirement in contract law. There are some concepts that relate to fairness, such as consideration and unconscionableness, but they are not a fairness requirement, there are fairness requirements in specific narrow cases (usually in agency situations), but it would be wrong to say that in the general case a contract has to be fair to be enforced.

None of this is legal advice, just broad overview for the purposes of a purely academic discussion. There are a lot of details that are omitted, so do not rely on anything said here but consult your own attorney instead.

How do you objectively evaluate "fair"? $0.30 cents per text isn't fair.

The agreement might be rolled into the one you agree to when you use the app.