Japanese crash course for new translator: お約束: SOmething happens without fail, like it was promised to happen (considering the people involved or situation before the occurence) 税込み: While it's "Before tax", it actually means "Tax included" as in "before you pay off the tax come with your payment/salary". And when used on merchandises, it's the original meaning, Which is "your payment includes the tax for this merchandise".
Presumably, because she's overly genki/ditzy and wouldn't remember to take off the tag and would run to show the admiral as soon as she bought it.
Commander: Whoa! It suits you well, Fubuki.Commander, how do I look in this new outfit?Tax includedThat's quite cliche of her...stareCommander is staring at me...