"Endlösung" is a specific euphemism used by the Nazis. Thus, it is part of the "Nazi language" and not used with its original meaning anymore. However, words that are merely similar or translate the same in English (why should that matter?) are not automatically Nazi language and avoiding them tends to be an overcorrection, at least if you are not writing a text where the context is problematic (such as a discussion of white supremacy or something similar). However, if you are in customer service, there is some likelihood that you will encounter someone who is overcorrecting, possibly even in bad faith. So, I understand why you might want to be extra careful. If that's the case I suggest using a synonym for "Lösung", which is easy to find in synonym dictionaries (e.g., see Duden) but depends on context.
Unfortunately, synonym dictionaries tend to be less than helpful for finding suitable alternatives for the complete term "Endlösung" because they focus on the actual meaning of the Nazi euphemism.
In general, "endgültige Lösung" is fine. Other alternatives are "Dauerlösung", "finale Lösung", "ultimative Lösung", ... However, with these, many educated Germans will recognize that you have looked for a synonym of "Endlösung" and that can create associations. Personally, I usually use synonyms of "Lösung" or avoid synonyms of "End-". I might say "beste Lösung", "tragende Lösung", ... but it really depends on context.
Another very good option is to switch from the noun to the verb. Instead of saying "Die <synonym of Endlösung> ist ..." just say "Das Problem wird durch ... endgültig gelöst". Usually, that's better style anyway.
