yuubinuke ni huzaihyou ga haitte ita node, saihaitatu wo onegaisimasita.

に marks the place where something ends up or exists. In 郵便受けに不在票が入っていた, it shows that the missed-delivery notice was in the mailbox.

Here, 入る means to go in / to be inside, and it normally takes に for the destination or location:

• 箱に入る = to go into a box
• かばんに入っている = to be in a bag

Using で would not work here, because で marks the place where an action happens, not the place where something is located as a result.

不在票 is the notice a delivery company or post office leaves when they tried to deliver something but you were not home.

It breaks down as:

• 不在 = absence / not being there
• 票 = slip / note / form

So it is basically a missed-delivery notice.

Because 不在票 is the thing that was in the mailbox, not the direct object of an action done by the speaker.

In 不在票が入っていた:

• 不在票 is the subject of 入っていた
• so it takes が

A very literal structure is:

• A が B に 入っている = A is in B

So:

• 不在票が郵便受けに入っていた = The missed-delivery notice was in the mailbox

入っていた is the past form of 入っている, and here it describes a resulting state: the notice was in the mailbox.

This is a very common use of 〜ている with verbs like 入る, 座る, 開く, etc., where it does not mean an action is actively ongoing, but rather that the result of the action is continuing.

So:

• 入った = it went in / it got put in
• 入っていた = it was in there

In this sentence, the important point is not the moment when the slip was inserted, but the fact that the speaker found it there.

Yes, 郵便受けに不在票があったので would also be understandable, but 入っていた sounds more natural and more specific.

Difference in nuance:

• あった = there was a missed-delivery notice in the mailbox
• 入っていた = a missed-delivery notice had been put in / was sitting in the mailbox

Because a notice is something that gets inserted into a mailbox, 入っていた matches the situation very well and feels more vivid.

Both ので and から can mean because, but ので usually sounds a little softer, calmer, and more explanatory.

So:

• 入っていたので、再配達をお願いしました = Because there was a missed-delivery notice in the mailbox, I requested redelivery.

Compared with から, ので often sounds a bit more neutral or matter-of-fact. It is very common in polite speech and explanations.

Roughly:

• から = because / since
• ので = because, with a softer explanatory tone

再配達 means redelivery or another delivery attempt.

It breaks down as:

• 再 = again
• 配達 = delivery

So after receiving a 不在票, you can ask the delivery company for 再配達, meaning they will try to deliver the package again.

お願いしました comes from お願いする, which means to request in a polite way.

So 再配達をお願いしました means:

• I requested redelivery
• literally, I made a request for redelivery

This is a very common polite pattern in Japanese, especially in service situations.

Similar examples:

• 確認をお願いします = Please check / I ask for your confirmation
• 対応をお願いしました = I requested assistance

It sounds more polite and softer than a more direct verb like 頼みました.

In this pattern, を marks the thing being requested.

So:

• 再配達をお願いしました = I requested redelivery

You can think of 再配達 as the content of the request.

This noun + を + お願いする pattern is extremely common:

• 返事をお願いします = Please reply
• 予約をお願いしました = I requested a reservation

Yes. Japanese often omits subjects when they are clear from context.

In this sentence, the natural interpretation is:

• I found a missed-delivery notice in the mailbox
• so I requested redelivery

So the subject of お願いしました is understood to be I.

Likewise, the person who put the notice in the mailbox is not stated, because it is not important here. The sentence focuses on what the speaker found and then did.

Because the speaker is talking about a completed situation in the past:

1. the missed-delivery notice was in the mailbox
2. the speaker requested redelivery

So both parts are past:

• 入っていた = was in there
• お願いしました = requested

Even though the notice may still physically exist somewhere, the sentence is narrating a past event, so past forms are natural.

It is basically a fixed compound noun meaning mailbox or letter box.

It comes from:

• 郵便 = mail / post
• 受け = receiver / receptacle

So literally it is something like mail receiver, but in actual Japanese it simply functions as the normal word 郵便受け = mailbox.

You should treat it as one vocabulary item rather than trying to translate each part every time.

It is polite but neutral, and it sounds very natural in everyday conversation or writing.

The politeness comes mainly from:

• お願いしました instead of a plain form like お願いした
• the use of 〜ました

Also, お願いする is a polite way to describe making a request, especially to a company or service provider.

So this sentence would fit well in normal polite speech, such as explaining what happened to a friend, coworker, or customer-service representative.