The longest job, and the one that produces the most mistakes.
Splitting the several hundred, sometimes several thousand prints and gift items that come back from the lab into individual orders is the easiest point in the whole process to get wrong. It is also where a mistake hurts most, because you are not the one who notices it. The parent is. Or you end up standing there at the end with a few prints and nowhere left to put them.
Every order has its own envelope
For a production run you print one envelope per order. It carries the order number and exactly what belongs in it, grouped by product, with quantities. The prints travel inside the envelope itself; for a gift item the same printed face goes on the side of the box.
There is no list to search through and nothing to hold in your head. What is on the envelope belongs inside it, and what is not on it does not. The numbering is simple and follows logically.
The envelope also says where it goes
If the package is going to a home address or a parcel locker, the delivery address sits at postal size in the middle of the envelope. If it is going to the school, the address is replaced by whose photos are inside, because handing them out there needs a class and a name rather than an address.
So the same envelope serves as both the packing list and the address. No separate label to print, and no way to mix the two up.
This is what makes it fast
In our own measurements a colleague of ours packed roughly 1,200 prints in an hour without a single error. Not because they pack especially fast, but because nothing has to be looked up, cross-checked or verified along the way.
Have a look at what the packing actually looks like.
With a real production run, and across different delivery methods too.