My first try is (When he had collected the parcel, he realized it was the wrong one.) My second is ( He had collected the parcel when he realized it was the wrong one.)
esalim80Can we also say ( He collected the parcel when he realized it was the wrong one.).
No. It implies he was a thief who wanted to steal a valuable parcel that belonged to someone else. So he collected it after he realized it was not his.