Lastly, like Jumper Geo implied, there's the possibility you don't receive exactly what you had in mind, like decanted liquid instead of sealed bottles, or less powder than targeted quantity (happened in the past), then the purity becomes paramount
Such a surprise may push you towards the edge to complete a decision with pressure sooner than wished for. For instance, if you were to change your mind with the will to postpone CTB, you could become without finances and soon or later without enough potent N.
Reciprocally, if in doubt of the N quality at reception, to gather equipment then send out sample and wait for lab test results, anything between 15 to 60 days could be necessary, depending on you (and abilities) - if relying on Exit, they send out their sampler kit with weeks' delay + if very urgent mail (cost) is sought, etc.
Plan it with some margin room ! ...and why not anticipate, seek testing equipment beforehand or at the same time
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Geo, I'm in western Europe. My own experience dates back 5 years ago, so not representative of the recent business climate (regarding potential geographical issues). I never use home address but general delivery (pickup for tourists) ...at a neighbouring country than mine, so I can then be super relaxed about not getting a visit