Stocked in advance?
"...the idea is to deliver items within minutes of an order. This suggests a significant quantity of shop stock is prepared in advance for drone deliveries..."
or rather, the pervasive and ever-growing Google ad network combined with their data tentacles reaching deep into the remotest crevices of the Internet, combined with a top-secret AI hidden deep underground (preferably a hollowed-out extinct volcano) will already know what you are going to order before you do, and have the necessary items on hand at the local depot