grammar of ～ちゃいましょう


I have a question about the following conversation.

> A: 駅はまだですか？ずいぶん歩いたみたいだけど。
>
> B: 近くの三つのデパートがバーゲン中で、こんなに混んでいるのですよ。
>
> A: そんなに歩きましたっけ。
>
> B: と、思うのですが、ほら、やっと交差点についたところじゃないですか。
>
> A: タクシーに **乗っちゃいましょうか** 。駅の近くにもデパートがあるから、もっと混んでいるかもしれないし。

I am wondering why A says "乗っちゃいましょうか" instead of "乗りましょうか". I know "～ちゃいましょう"
is the shrinkage of "～しまいましょう" and "～しまう" is used to show "astonishment". In
my grammar book, there are only past tense sentences with "～しまう".

Under what kind of situation would a Japanese native speaker use "～ちゃいましょう"
rather than the plain form of "～ましょう"?




The nuances of 「Verb + て + しまう」 can be fairly subtle, but they are important
as the native speakers do use this phrase on a daily basis.

In this context, the nuance is that the speaker and listener think that it is
**_against their (original) will/plan_** to take a cab, but they would
probably have to do so for their convenience on that particular day because
the sidewalks are extremely crowded.

If, instead, 「乗りましょうか」 were used, it would sound as if taking a cab was as
natural (and usual) a choice as walking for the speaker. It would tend to be
used by people who often use a cab as a method of transportation even for
short distances.




The nuances of 「Verb + て + しまう」 can be fairly subtle, but they are important
as the native speakers do use this phrase on a daily basis.

In this context, the nuance is that the speaker and listener think that it is
**_against their (original) will/plan_** to take a cab, but they would
probably have to do so for their convenience on that particular day because
the sidewalks are extremely crowded.

If, instead, 「乗りましょうか」 were used, it would sound as if taking a cab was as
natural (and usual) a choice as walking for the speaker. It would tend to be
used by people who often use a cab as a method of transportation even for
short distances.

