Well, if you see it from the perspective of a corporation then procurement is probably one of the fields most ripe for fraud.

If you're just able to hand in an invoice to accounts payable then it's very, very easy to come up with bogus invoices. In fact this used to be a quite common scam.

Thus those rigid controls on all things procurement.

I worked for a variety of global companies and government entities. Setting up the whole invoicing process is generally a pain and usually you'll have to wait for a couple of month until the process works flawless.

Once it works it usually does work flawless.

I'm not arguing that this is great. But I can understand where a company is coming from, since the whole field of procurement is just a huge invite for fraud and abuse, if it's not rigidly controlled, which, I'm afraid, also involves an approved supplier list.

[EDIT : Spello corrected]

Indeed, a better way to fight this would be to start making the cost visible to your manager. Just start a spreadsheet, "item cost" vs "total company cost", resulting in "Cost overhead". Just hand these to your boss each month. No snarky comments, no aggression, just "Look, I have gathered some interesting information".

Once he sees a rather ridiculous cost overhead right there on paper and ideally even verfiable with your orders through other systems, interesting things happen. More often than not, management didn't know the excess of the situation.