syanpuu no tumekae wo kai ni ittara, senzai mo tokubai desita.

詰め替え means refill.

In シャンプーの詰め替え, it refers to a refill pack of shampoo, not a new full bottle. In Japan, many products are sold this way, so 詰め替え is a very common word on packaging.

It comes from the verb 詰め替える, which means to refill or to transfer into another container.

The の links the two nouns and shows their relationship.

So:

• シャンプー = shampoo
• 詰め替え = refill
• シャンプーの詰め替え = a shampoo refill / refill for shampoo

You can think of の here as something like of or for, depending on what sounds natural in English.

Because with the pattern verb stem + に行く, Japanese uses the stem of the verb, not the dictionary form.

For 買う:

• dictionary form: 買う
• stem: 買い

So:

• 買いに行く = to go to buy
• 買いに行った = went to buy

This is a very common pattern:

• 食べに行く = go to eat
• 見に行く = go to see/watch
• 飲みに行く = go to drink

It means to go somewhere for the purpose of doing something.

So 詰め替えを買いに行った means:

• went to buy the refill
• more literally, went in order to buy the refill

The に here marks purpose.

A similar pattern also works with 来る:

• 買いに来た = came to buy
• 見に来た = came to see

The を marks the direct object of 買う.

So:

• 詰め替えを買う = buy the refill

Even though 買う appears as the stem 買い before に行った, the object marker still belongs to 買う.

So the structure is basically:

• [詰め替えを買い] に 行った
• went [to buy the refill]

たら is often taught as if, but in real Japanese it also very often means when or after in past narrative contexts.

Here, 買いに行ったら means something like:

• when I went to buy it
• when I went there to buy it
• after I went to buy it

The nuance is that the speaker went somewhere, and then discovered the next thing: 洗剤も特売でした.

So in this sentence, たら is less like a hypothetical if and more like a when/and then I found that... meaning.

This is completely normal in Japanese.

Subordinate clauses often use plain form even when the main sentence is polite. So:

• 行ったら = plain form inside the clause
• でした = polite ending of the main statement

Japanese usually marks politeness mainly at the end of the sentence, not in every part of it.

So this combination is natural:

• 買いに行ったら、洗剤も特売でした。

も means also or too.

So 洗剤も特売でした means:

• detergent was also on sale
• the detergent was on sale too

The idea is that the speaker went for one thing, but found an additional good deal.

Without も, it would just say:

• 洗剤は特売でした or 洗剤が特売でした which would not emphasize the also/too meaning in the same way.

特売 is a noun meaning special sale or bargain sale.

In this sentence, 特売でした works like:

• was on special sale
• was a sale item

Japanese often uses nouns with だ / です this way.

So:

• 特売だ = is on special sale
• 特売でした = was on special sale

It may feel a little different from English, because English would often use an adjective like cheap or a phrase like on sale, but Japanese commonly uses the noun 特売 this way.

洗剤 is a broad word meaning detergent or cleaning agent.

Depending on context, it can refer to things like:

• laundry detergent
• dishwashing detergent
• other household cleaners

In everyday shopping contexts, many learners will most often see 洗剤 used for laundry detergent, but the word itself is broader than that. The exact kind is understood from context.

Yes. The subject is omitted because it is obvious from context.

A natural English translation uses I:

• I went to buy a shampoo refill, and detergent was also on special sale.

Japanese very often leaves out subjects like I, we, or it when they are already understood. That is one reason Japanese sentences can feel more compact than English ones.