“Because you have to”
If one of the line items on your invoice is for “Microsoft SQL Server 2005”, and it’s £900/pa more than I can buy it for directly from Microsoft, you really should have a good reason for why I should buy it from you.
“Because you have to” is not a good reason.
It is the very opposite of a good reason.
The software is the same regardless of who I buy it from. The company in question here actually isn’t ripping me off intentionally; I’m pretty sure they simply don’t get the educational discounts that the school does when buying direct. However, I am now eagerly awaiting a call from someone more senior to hear their allegedly sound reasoning about why I have to buy a generic software package at their rates instead of getting the exact same thing directly from Microsoft for a fraction of the price.