Shizuka is a dumbass
She went there to buy "special underwear" but starts absolutely tweaking out when swimsuits were brought up
So she's fine with literal lingerie but drew the line at swimsuits........ight
What I could infer is the special underwear will at least be planned to be shown to Souma only, while swimsuits will be visible in public and there may be some... "comparisons" 🍈🍈

between Shizuka and her peers. She could be also be shy because she'll be shopping with others she know compared to just by herself (and the store clerk).
But I could agree with you she (or the author, really) is just being a dumbass as usual.