The CYA thing to do here is to create a paper trail of you asking your boss and HR what procedure you need to follow to determine if something is essential. They likely won't respond at all or in writing, send a follow-up in writing noting their lack of response.
This is the true bureaucratic answer. Also insert a CYA about why you're signing the form now, so you can keep signing it after their non-response without exposing yourself to too much risk. Something like, "I've attempted to verify the list of items, and to my knowledge they appear essential, but can you clarify the procedure for determining whether an expenditure is essential or not?"
Do you really send two or more emails for every piece of red tape that you go through? The overhead here is ridiculous, all for CYA from lawsuits that won't affect 99% of workers.
