Wow! $300 is a lot of money! We only get $40 to spend, and we only have 3 choices of where to spend it.
I agree with you in that you can some items cheaper somewhere else, but it is most likey easier for the person in charge of money orders/budget to keep up with the paperwork if it from one place. I'm guessing that is why you might be told to only shop at Lakeshore.
Since that $300 is not coming out of your own pocket, I guess just smile and spend!
I, also, used to spend a lot out of my pocket on my classroom for years and years. After over 30 + years....spending $200 per year adds up to $6,000... I wish I hadn't done it