A few random ideas to help out:
if you are confident enough, collect a list of things to buy, links and such
for example, "cool pens, tshirt, notecards, cool socks"
show them everything, and if they ask be like, "hey, I saw these socks and they look cool, can I get them?"
do it super low-key and casually, especially if it is your own money.
or, if you are into cosplay, say its for that, and just wear it normally as well.
you could use the "i'm cold but don't like pants" excuse (i often wear basketball shorts + long socks)
