You probably are worrying too much. The rules of usage and ettiquette are far more liberal than they used to be. However, some people still prefer to be more formal, so it is a good idea to incline towards formality. You can then see how the person you wrote to addresses you, and respond similarly.
With Ms Smith you are as safe as you can hope to be.
The only degree that is accompanied by a prenominal title is a doctorate. So, Ms Smith remains Ms Smith whether she has a Bachelor's and/or Master's degree, or any form of certificate or diploma.