Christopher Nkunku's first-half strike put PSG on track for three points to round off an underwhelming ICC campaign that had seen Tuchel's men lose to Bayern Munich and Arsenal.
When Moussa Diaby added a drilled second PSG looked set to close out an easy victory ahead of its competitive 2018-2019 bow against Monaco in the Trophee des Champions.
Atletico beat Arsenal on penalties in its last ICC match and Diego Simeone's side looked set to have the chance to repeat the trick after coming from two down in Singapore.
Victor Mollejo finished off a rebound with 15 minutes to go, giving Atletico hope of a fightback, which was seemingly completed when Antoine Bernede diverted a low cross into his own goal.
But in the second minute of added time, teenager Postolachi bent a brilliant left-footed curler beyond the despairing dive of Antonio Adan to ensure PSG ended its ICC campaign on a high.