See the DSUS Director of R&D's earlier video of the Payloader system (since developed further for ProDropper). In the first 1.5 minutes you see:

https://www.youtube.com/watch?v=-WTqnCmdKX4

An electromagnet is fixed to the drone.

A simple metal plate is permanently attached to a webbing strap which goes around the circumference of the payload. This metal plate interfaces against the electromagnet and is held firmly in place until the release signal is sent by the operator.

Not sure what is visible in that picture on the website - looks the black webbing strap is not as wide the orange one in the video and perhaps some additional thin binding has been used towards either end of the bottles to make the qty 4 more stable together before the web strap is put around them.

The key points are that the basis of the system is an electromagnet and the "disposable" / "reusable" items used for each drop are cheap.