here is my thoughts on it, Depending on your shop where he works and you get your rodents from, it could simply be a ploy to keep you purchasing mice where the profit margin is a lot higher. i will just use my local numbers here on this.
you come in asking about rat pups, which will lead into larger rats for food. locally for me a small rat (sorry no pinks, fuzzies or hopper rats) are $5 each. where as a adult mouse is $5.99 each, knowing 1 Small rat will feed a snake for a good week or more for the average pet BP, a snake able to take a small rat cold easily eat up 2,3, or more adult mice at a meal.
with my average order of 200 rodents per order from Rodentpro, Including the shipping fee's, it roughly breaks down to about $0.72 an adult mouse, and about $1.10 for a small rat. not sure what your local store is getting them for, or re-selling them for. but a marked price from $0.72 to $5.99 for an adult mouse is a massive profit margin.