Yes, "smth" does mean "something", but you’re right - it’s best not to use it in business correspondence. It is more suitable for a conversational tone. Here is good article that explains in detail the meaning and examples -
https://it-rating.com/it-articles/what-does-smth-mean-and-how-do-you-use-it. It states that "smth" is often used for informal messages where speed and ease of communication are important. So if you talk to colleagues as a friend, you can, but in letters to customers - better full "something".