Non-Drop is the provider's expectation that the delivery holds. It's a forecast, not a commitment — nothing happens automatically if it turns out wrong.
Refill is the actual guarantee. Within the stated window you can request a top-up and it gets delivered again, no argument.
A service marked Non-Drop with
no refill window gives you nothing to fall back on. A slightly pricier service with
Refill 30 Days is usually the cheaper one after drops.
Refill counts from the order date, not from the day you notice the drop. Check your orders before the window closes.