No body knows why there is no text message add-on.
The $10 50 CW minutes + 50 outgoing texts + free incoming texts
If you need more than 50 outgoing text, you need to select other plan.
@Horemansuk as @popping mentioned, literally EVERY other plan except the $10/50/50 is avaialbe with either no texting at all or completely unlimited texting. For whatever reason, PM has not [yet] decided to offer domestic texting as an add-on, so if you need more than 50 you may want to pick another plan.