A lot of buyers will have something like this as a default "note to seller", as in it is something they just automatically include with every purchase they make, regardless of any postage services selected (most assume the default is parcel, though, no matter how much the postage cost is).

They generally just want to make sure the safe-drop location is consistent and / or to ensure it's not returned for collection at a PO (basically, it's another way of requesting no signature on delivery without being explicit, which can ring alarm bells for sellers). I don't even acknowledge it most of the time, and will not include it on anything going as a letter (no point, they will go to letterboxes only, and if too big will just be placed near it 99% of the time - the only time I include buyer's instructions that are not part of the official address is when they are there to assist in locating the premises itself, eg "corner of X and X" or "down the lane behind the X"). A couple of times they've sent me a message instead, and in those cases I just reply that delivery instructions are not applicable to letters as they will always deliver directly to a letterbox.