okurisaki no yuubinbangou wo hitotu matigaete, sukosi awatemasita.

送り先 means destination / recipient address / where something is being sent.

It is often used when talking about mailing, shipping, or delivery.
By contrast, 住所 means address in a more general sense.

So:

• 送り先 = the place/person you are sending something to
• 住所 = an address in general

In this sentence, 送り先 fits well because the topic is a postal code for something being sent.

Here, の links two nouns and shows that the second noun belongs to or is associated with the first.

So:

• 送り先 = destination / recipient
• 郵便番号 = postal code
• 送り先の郵便番号 = the destination’s postal code / the postal code of the place it was being sent to

This is a very common noun-connecting pattern in Japanese.

を marks the direct object of the verb.

The verb here is 間違える, which means to make a mistake with, to get wrong, or to confuse something.

So in:

• 郵便番号を間違えて

the postal code is the thing that was gotten wrong.

You can think of it as:

• I got the postal code wrong

Yes, literally 一つ means one thing / one item, but here it is being used more naturally as one part, one digit, or one spot.

In this sentence, it most likely means the speaker made one mistake in the postal code—very often, that would mean one digit was wrong.

So 郵便番号を一つ間違えて means something like:

• I got one part of the postal code wrong
• I made one mistake in the postal code
• I entered one digit incorrectly

Japanese often uses 一つ in this broad way when counting mistakes or small units.

間違えて is the て-form of 間違える.

The て-form is being used to connect actions:

• 送り先の郵便番号を一つ間違えて、
• 少し慌てました。

This means:

• I got one digit of the destination postal code wrong, and then I panicked a little.

So the first action is put in て-form to link it to the second action.

If you said 間違いました, it would sound like a separate finished sentence unless you added another connector.

It usually means to make a mistake with something or to get something wrong.

Examples:

• 名前を間違える = get the name wrong
• 番号を間違える = get the number wrong

Be careful not to confuse it with expressions like:

• 間違っている = to be wrong / mistaken

In this sentence, 間違える is transitive, so it takes を:

• 郵便番号を間違える = to get the postal code wrong

少し means a little.

It softens the statement and shows the speaker was only somewhat flustered, not extremely panicked.

So:

• 慌てました = I panicked / I got flustered
• 少し慌てました = I panicked a little / I got a little flustered

Leaving it out would make the reaction sound stronger and more direct.

慌てました comes from 慌てる, which means:

• to panic
• to get flustered
• to get into a hurry because of surprise or worry

It often suggests a brief emotional reaction, like realizing something is wrong and feeling unsettled.

So in this sentence, it does not necessarily mean full-blown panic. It can mean:

• I got a little flustered
• I was a bit alarmed
• I panicked a little

The subject is omitted because it is understood from context.

Japanese often leaves out subjects when they are obvious. Here, the natural implied subject is I.

So the full idea is:

• I got one digit of the recipient’s postal code wrong and got a little flustered.

This omission is extremely common in Japanese and does not make the sentence incomplete.

Yes, 郵便番号を一つ間違えて is natural.

Here, 一つ modifies the idea of how many mistakes / parts were wrong, and it appears before the verb 間違えて.

So the structure is roughly:

• postal code + object marker + one thing/one part + got wrong

Putting 一つ before 郵便番号 would sound less natural if the intended meaning is made one mistake in the postal code.

Compare:

• 郵便番号を一つ間違えて = got one part of the postal code wrong

That is the most natural reading here.

Usually, no. In this context, it most naturally means I entered or wrote part of the postal code incorrectly.

While 間違える can sometimes involve confusing one thing with another, with 番号 it often simply means:

• using the wrong number
• writing a number incorrectly
• making a mistake in the number

So the most likely interpretation is:

• I made one mistake in the recipient’s postal code

Yes. It ends in ました, which is polite past tense.

• 慌てました = polite
• casual version: 慌てた

So this sentence is suitable for normal polite conversation or writing.

If made fully casual, it could be:

• 送り先の郵便番号を一つ間違えて、少し慌てた。