I've rarely encountered situations where a superior wouldn't tell me on the record exactly what level of diligence is necessary, and the few exceptions turned out to be extremely toxic situations.
Which expenditures are "essential" can't always be objectively determined anyway. The purpose of the rule is not to ensure that inessential expenditures don't slip through the process but rather to ensure a chain of accountability that can be checked if some flagrant violation is discovered. In the letter-writer's position, it's probably only their job to be able to produce evidence of who assured them that an expenditure was essential, which means they need to put the right language on the form by which people request expenditures. As far as I understand it, procurement teams exist because a dedicated team of specialists can be more efficient and effective at procurement, not because spending decisions should be second-guessed by people who don't have the background to understand them.
Which expenditures are "essential" can't always be objectively determined anyway. The purpose of the rule is not to ensure that inessential expenditures don't slip through the process but rather to ensure a chain of accountability that can be checked if some flagrant violation is discovered. In the letter-writer's position, it's probably only their job to be able to produce evidence of who assured them that an expenditure was essential, which means they need to put the right language on the form by which people request expenditures. As far as I understand it, procurement teams exist because a dedicated team of specialists can be more efficient and effective at procurement, not because spending decisions should be second-guessed by people who don't have the background to understand them.