Why couldn't he just say they were for a patient of his
As her doctor he should already know her sizes, but finding clothes that properly fit her body, and complements her looks is another demon. And it’s not like she can try them on before they’re purchased. He basically lucked out.
Realistically for a man, buying women’s clothing and other feminine products in a store that was designed for women customers only, with no female relative, confidant, or lover for extra help, is beyond nerve wracking! A little less so if he was shopping at a general clothing store, but the pressure is still intense.
Stating your true intentions for making the purchase won’t actually stop people from giving strange looks, having awkward smiles, thinking you’re lying your perverted ass off, having bad impressions of you, snide comments from kids, and/or worse giving “encouraging words” that don’t feel encouraging at all…It’s hard to tell if people genuinely understand your cause when you’re under pressure.
Don’t forget he lives in a world where slavery is normal.