Delivery drivers and parcel throwing

[Spectre]

I've just participated in a newspaper thread about parcels being thrown over a wall. I see them quite often, maybe due to the current conditions.

Here is my offering to mankind! (I'll not be able to afford the bill for the patent, I bet it exists anyway...).

"ShockWatch" type sticker with barcode/QR at the back. Once activated, the liquid goes opaque, the barcode cannot be scanned.