When using direct object pronouns that end with a vowel:
te ----> (NOT "tu")
there should always be an elision.
The reason "Tu es bête" is not written (at least formally) "T'es bête" is because it is constructed with the subject pronoun Tu + verb.
You would never write (again, formally or academically):
T'envoies une lettre à... ?
T'oses faire ça...
because these are examples of where "Tu" is the subject, but you could say
Il t'envoie une lettre...
On ose te faire ça...
because here, "te" is the direct object pronoun and sometimes requires an elision.