sokukounyuu no ato, konpou wo sumase, hassoutuuti wo okutta.

Because 即購入 can function as a noun here.

• Noun + の後 = after N
• Verb + た後 = after doing V

So both of these are possible in principle:

• 即購入の後
• 即購入した後

The version with の後 treats 即購入 as an event or thing: after the immediate purchase.
The version with した後 focuses more directly on the action: after someone bought it immediately.

Since 購入 is a suru-noun, Japanese can often use it either as a noun or as a verb phrase depending on the structure.

即購入 is a common online marketplace expression. It usually means buying immediately, often with the nuance of buying without asking first or without prior discussion.

It is especially common on resale apps and auction-style platforms. So the nuance is not just quick purchase in a general sense, but often something like:

• an immediate purchase
• a buy-it-now purchase
• purchasing right away without messaging first

In ordinary non-marketplace Japanese, people might more naturally say things like すぐ購入する or すぐに買う, but 即購入 is very natural in online selling contexts.

Here, 後 is most naturally read あと:

• 即購入の後 → そくこうにゅうのあと

The reading のち is also possible in some formal or literary contexts, but in a sentence like this, あと is the normal reading.

Because 後 is being attached to a noun phrase.

The basic pattern is:

• noun + の後 = after noun

So:

• 即購入 = a noun-like expression
• 即購入の後 = after the immediate purchase

If you use a verb instead, you do not use の. Instead, you use the past form:

• 購入した後
• 買った後

So the の is there because the thing before 後 is being treated as a noun.

済ませ is the conjunctive form of 済ませる, and in this sentence it connects to the next action. In everyday speech, many learners will first meet this as 済ませて.

So these are very close in meaning:

• 梱包を済ませ、発送通知を送った。
• 梱包を済ませて、発送通知を送った。

The version without て sounds a bit more concise and written. You often see this style in:

• written reports
• business writing
• product or transaction descriptions
• narrative summaries

So 済ませ、 is not strange; it is just a slightly more formal or compact written connection.

済ませる adds the nuance of finishing or getting something taken care of.

Compare:

• 梱包した = packed it
• 梱包を済ませた = finished the packing / got the packing done

So 済ませる emphasizes completion. It suggests the packing was not just done as an action, but completed as a task.

That is why it fits well in a sequence like this:

1. purchase happened
2. packing was completed
3. shipping notice was sent

Because 梱包 is the direct object of 済ませる.

The pattern is:

• X を 済ませる = finish X / get X done

So:

• 梱包を済ませる = finish the packing

Even though 梱包 can also be used with する as a verbal noun, here it is being treated as the thing that gets completed, so を is used.

Because 発送通知 is the thing being sent.

The pattern is:

• X を 送る = send X

So:

• 発送通知を送る = send a shipping notification

In other words, 発送通知 is the object of 送った.

Yes, it is understandable and basically natural, especially if the context is a system message, marketplace notification, or seller-side transaction flow.

That said, depending on the exact situation, native speakers might also say:

• 発送通知をした
• 発送連絡をした
• 発送通知を送信した

There are slight differences:

• 発送通知を送った sounds like sent a shipping notification
• 発送通知をした sounds like issued/gave shipping notice
• 発送連絡をした sounds like contacted the buyer to say it was shipped
• 発送通知を送信した sounds more explicitly digital/system-based

So the original is fine, but other phrasings may sound more natural depending on platform and context.

The subject is omitted, which is very normal in Japanese.

From context, the most likely subject is the seller or speaker, so in English it would often become something like:

• I
• the seller
• we (if a business is speaking)

Because Japanese often leaves out subjects when they are obvious from context, the sentence does not need to say I explicitly.

Yes. It strongly suggests a sequence:

1. 即購入の後 = after the immediate purchase
2. 梱包を済ませ = finished the packing
3. 発送通知を送った = sent the shipping notification

So the sentence presents the actions in chronological order. The connective form 済ませ、 links one completed action to the next.

送った is the plain past form of 送る.

• dictionary form: 送る
• plain past: 送った

This makes the sentence sound like a plain-style statement or narrative sentence.

A polite version would be:

• 即購入の後、梱包を済ませ、発送通知を送りました。

So the grammar is not especially casual, but the final verb is in plain style rather than polite style.

Yes, 即購入後 is possible, especially in more compact or formal written language.

Compare:

• 即購入の後 = more standard and transparent
• 即購入後 = more compressed, label-like, or formal

You often see the shorter compound style in notices, business writing, or headings. For learners, 即購入の後 is usually easier to understand because the structure is clearer.

No. Standard Japanese normally does not use spaces like this.

It would usually be written as:

即購入の後、梱包を済ませ、発送通知を送った。

Spaces are often added only in teaching materials to make the sentence easier to read for learners.