Use the item for those purposes, only.
Use the item for those purposes only.
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There's no need for the comma because that would imply that you're starting a new phrase, but you're not. Also, when you say the sentence out loud, you don't pause after "purposes".
Commas don't indicate pauses but parts of a sentence, usually. You can't use "I pause when I come to this point in the sentence" as a rule for when to use a comma.