[Thermodyne]

I would address it to Jane Smith. And start the letter with just "Jane"

If I am paying them for the service, then I want to be sure that they know their place in the pecking order.

Example:

Jane,

I am very upset with the results of the service call preformed by your techs last week. As you well know our contract specifies that the service techs will provide all parts, tools, and supplies yada yada yada


For a more polite letter it's Ms. Unless you have been introduced or are replying to correspondence that was signed Mrs.