Undocumented charges tend to surface at the worst possible moment — after a place has been promised, before anything is signed — when querying them feels like risking the flat. The useful question is not whether the amount is large, but whether the landlord did anything specific to earn it.
Probably not, unless it corresponds to something concrete the landlord actually did for your specific benefit. Dutch law bars charges that give one party an "unreasonable advantage" over the other, and drafting a standard contract or doing routine paperwork counts as ordinary cost of renting out a property — not a service you should be billed extra for. Courts have repeatedly struck down "administratiekosten" and similar line items precisely on that basis. The fact that it's undocumented in your contract makes it weaker still, since there's nothing you agreed to that spells out what you're paying for. Ask for a written breakdown of what the fee covers; if the answer is vague or it's just "processing," you're within your rights to refuse it or claim it back if you've already paid.