Either is fine. "Had returned" is a bit clearer and more formal/educated. Except on rereading the paragraph, I see that "had" was used in the previous two sentences, so I'd say use "had" in the third sentence as well.

I guess the correct answer in this instance would be the Pluperfect tense (neither perfect nor simple). The sentence in question should read "Luckily, he had returned it to us." The perfect tense would be "he has returned it to us" which is a little too close in the past in comparison to the event described. The past simple tense is also not far back enough in the past as your story references a meta-event in the even more distant past. Easiest way for you to know this is to check for consistency. :) You used pluperfect here: "someone *had put* our credit card in our mailbox." and here: "our neighbour who had seen it". The returning action took place in this same time period. Therefore, it makes sense that the tense should remain consistent. :)

Hmmm. I think the natural choice here is the past perfect; "Luckily, he had returned it to us." This keeps us in the same narrative flow as the rest of the piece. There is an argument to be made, though, that we are stepping out of the past and into the present at the end of the little narrative here: our good luck in getting our card back continues beyond the end of the story and into the present. So I don't think it would be any kind of an error to choose the simple past. Then the piece might continue: "And that means I can pay for the pizza after all." "Oh, here's the bill. I found I'd lost my credit card when Tim and I were out shopping yesterday. After we got home, we found that someone had put our credit card in our mailbox. It was a big surprise to discover that this person was our neighbour who had seen it on the asphalt in front of our house. Luckily he returned it to us. So I can pay for this!"