The messages exchanged using SMS/MMS are almost always in plain text which makes them inappropriate for confidential transactions. Consider writing an email instead.
These messages travel through the networks in plain text, and copied along the way as part of the transmission process. You have no control over the message as it traverses the networks. You may consider limiting the amount of sensitive information you include in these messages.
They also may reside on your device (as well as the destination device). You can design and agree to a process of regularly deleting such messages you send (or receive).