Learn Hardcore Japanese: hassou kara toutyaku made no kikan ha mikkainai ni narimasu. - The period from shipping to arrival will be within three days.

から marks the starting point and まで marks the endpoint. So 発送から到着まで means “from shipment to arrival.” They commonly appear as a pair, but you can drop one if context makes the other obvious (e.g., 到着まで “until arrival” if the start is already known).

Why is there a の before 期間? What does 到着までの modify?

の turns the preceding phrase into a noun modifier. 到着までの期間 literally means “the period up to arrival.” Here, the whole chunk 発送から到着までの modifies 期間 (“period”).

What does the topic marker は after 期間 do? Could I use が?

は sets 期間 (“the period”) as the topic: “As for the period from shipping to arrival, …” Using が would sound off here because we’re not introducing new, contrastive information about the subject; we’re describing a known topic. は is standard in such explanatory/business statements.

Why use になります instead of です or かかります?

です: neutral statement (“It is within three days.”).
になります: polite, somewhat softer/announcement-like (“It will be/comes to be within three days.”). Very common in customer-facing Japanese.
かかります: focuses on time required (“It takes up to three days.”).

All are grammatical; choose based on tone. In notices, 〜になります/〜となります sounds formal and non-assertive.

What is the に doing after 三日以内?

With なる, the resulting state takes に: “become X” = X になる. So 三日以内になります = “(it) becomes ‘within three days’.” If you use a verb of occurrence (e.g., arrive), you’d say 三日以内に到着します (“arrives within three days”), where に marks the time limit “by/within.”

Does 三日以内 include the third day? How is it counted?

以内 is inclusive of the upper limit: “within three days” means “in three days or less.” How you count from shipment can vary by company:

Linguistically, it means within 72 hours of shipment, including the 3rd day.
Practically, businesses often count in calendar days and may start from the day after shipment.

To be explicit:

発送日を含めて三日以内 = including the shipping day
発送日の翌日から起算して三日以内 = counted from the day after shipping

For business days, use 営業日 (see below).

Do I need 間? What’s the difference between 三日 and 三日間? Is 三日間以内 okay?

三日 and 三日間 both mean “three days (duration),” but 三日間 highlights the span more explicitly.
With 以内, the natural form is 三日以内. 三日間以内 is sometimes seen but often avoided as redundant or stiff in everyday usage. Prefer 三日以内.

How do I read 三日 here—mikka or san-nichi?

Read it みっか (mikka). The reading さんにち (san-nichi) is mainly used inside set compounds (e.g., 三日天下) and not for plain durations. 三日間 is みっかかん (mikkakan).

Could I use 届く instead of 到着? Any nuance difference?

到着する: “arrive” (neutral, general arrival at a place).
届く: “be delivered/reach the recipient” (focus on delivery completion).

For parcels to a customer, 届く/お届け often feels more customer-oriented. Example: 発送から三日以内に届きます.

Is 発送 the same as 出荷, 配送, or 送付?

発送: sending out (mailing/shipping) from the sender’s side; common for parcels.
出荷: “shipping out” from a factory/warehouse (inventory/logistics term).
配送: distribution/delivery process by a carrier.
送付: sending (often documents/items) in formal/business contexts.

Choice depends on context; your sentence is fine with 発送.

Can I drop 期間? Is 発送から到着までは三日以内です natural?

Yes. Alternatives:

発送から到着までは三日以内です。
発送から到着まで三日以内です。 (topic understood)
Keep your original with 期間 for a more formal, written tone: …の期間は…

Can I say 三日以内で here? What’s the difference between 以内に and 以内で?

For time limits, use 以内に with event verbs: 三日以内に到着します.
以内で is more natural with quantitative limits on means/amount: 1000円以内で買えます. For time, 以内に is the safer default.

How would I say it more directly as a delivery promise?

発送から三日以内に到着します。
発送から三日以内に届きます。
Polite/customer-facing: 発送から三日以内にお届けいたします。
If it’s an estimate: 発送から三日以内に到着する見込みです。

How do I say “within three business days”?

Use 営業日:

発送から三営業日以内に到着します。
ご注文後、三営業日以内に発送いたします。 Note: 営業日 excludes weekends/holidays unless specified.

Why are there spaces between the words? Japanese usually doesn’t have them, right?

Correct—standard Japanese doesn’t insert spaces. The spacing here is a learning aid to show word boundaries: 発送｜から｜到着｜まで｜の｜期間｜は｜三日以内｜に｜なります.

What’s the register and tone of this sentence? Is it suitable for customer-facing text?

It’s polite and businesslike. 〜になります and the nouny phrasing (…の期間は…) feel like a notice or FAQ entry—appropriate for websites, storefronts, and emails.

Is 以内 the same as 以上 or 以外? I mix them up.

以内: within, not exceeding (≤). Example: 三日以内.
以上: greater than or equal to (≥). Example: 三日以上 = three days or more.
以外: except for/other than. Example: 三日以外 = other than three days.

Any small pronunciation tips for this sentence?

発送 = はっそう (hassō) with a doubled “s” (sokuon っ).
到着 = とうちゃく (tōchaku), long “ō” in とう.
期間 = きかん (kikan).
三日以内 = みっかいない (mikka inai); link the vowels smoothly: mi-kkai-nai. Say it as: ha-ssō kara tō-chaku made no ki-kan wa mik-ka i-nai ni na-ri-masu.