Cashier switched my size

the cashier wanted the same size of underwear that I had and decided to switch sizes giving me a medium not small. Pity that Mr Price doesn't have a return policy on underwear. So I'm stuck with a size of undies I'll never wear. I've been a fan of Mr Price for years but now, the cashier Naomi changed all that. I guess the customer is always right doesn't work there. I also contacted support talking with Jessica Moolman because I realized this after got home but there was no way forward.