1. These things rarely are effective.
2. If sealer is detected in system, auto a/c shop will not work on it because the sealer will destroy their multi thousand dollar recovery/recycle machine when it is recovered.
3. One is rarely successful in getting "something for nothing" which is what such products offer.
I know it's a hard pill to swallow, but buy an evaporator and change it yourself. There are several threads regarding the job. A fellow from Houston that I have met recently changed it and posted afterwards that you should not be afraid of the job.