How does one manage the shifts of Channel Balances in the Lightning Network
It's actually a non problem deliberately set up in this video as a strawman. Professionally produced FUD.

In practice you're very unlikely to have a direct channel with the specific store in the first place, since for privacy reasons you actually want to be separated from that store at least 3 hops. But in either case you'd simply route your payment in the other direction through a different route towards that store. That's the whole point of this thing being a network. Payments can flow in both directions and the network is full of cycles.