When I use A/C everything work fine, but 30 sec. after it's turned off, it begins to stink really bad. What do I do ? Change expensive charcoal filter or clean the evaporator ? Service supposedly doesn't change them under warranty. Is that true ?

Charcoal filters . . . there's two!
 

As I recall, the pre-filter get serviced at 90K or 100K miles. But don't recall seeing anything about the main filter.

Just changed my S500 charcoal filters (95K miles); the pre-filter, which is easy, cost about $100.

And the main filter. This is difficult to get out. Cost also about $100.

But no stink! The prefilter was very WET because the air plenum drain hose got diverted and drained into the the pre-filter cavity. It had a damp smell but was not detectable in the car.

Good luck, it's a job.

You may have a dead mouse in there somewhere.

Bleach
 

I work at a Ford garage, when we have "stinky" AC problems, we remove the evaporator and flush the inside and outside with bleach and a Motorcraft 'disodorizer' part # YN-18. Available at any Ford store for about seven bucks. Its a pain and it sucks but it does work.