Should we assume no diagnostic codes (check engine A1) came up for the trans? The codes that can become set cover many of the more readily repairable incidences. You might have already changed the fluid, but was the old fluid burnt? And does the transmission slip all the time or only sometimes, like when very warm or very cold? All of this is just to help sort out things short of an unrepairable gearbox. If it was my car and I was not going to work on it myself I would definitely follow the others' advice to get a new opinion.