Which sentence or sentences have correct parallel structure?
kelsey, after you confirm that our mailing list is up to date, alan will stuff
the envelopes, address them, and drop them off at the post office.

Alan will stuff  the envelopes, address them, and drop them off at the post office

Explanation:

Parallelism or Parallel structure makes an effect on the reader by making a balance in one or more sentences that have same grammatical structure. In this structure, it uses repetition of a grammatical form (same pattern of words) in one sentence. For example, in the sentence, Radha likes cooking, reading and playing guitar. This is using same form of grammatical structure like cooking, reading and so on.  

In the sentence, Alan will stuff the envelops, address them, and drop them off uses same pattern of words and creates parallel structure.