Maybe email her and just politely advise that due to several orders having "missing" items could she photograph the next order package she receives before opening it. At least that way you should be able to tell if it has been tampered with, although if she is simply stealing the items she claims are missing it wouldn't be hard to open the parcel take out a couple of items and reseal it, then photograph to show its been tampered with.

it might be worth contacting AP and letting them know that this person has claimed missing items several times and is there anyway they can put some kind of "watch" on her parcel deliveries.

i guess at the end of the day the only other option is send all her orders separately without combining them.