A threshold set at a round number rewards customers who were already spending that much.
Thresholds should come from your actual order value distribution, not a round number, or you discount behaviour you already had.
The progress indicator is the mechanic — a shopper who cannot see how close they are has no reason to add anything.
Too many tiers reduces response; two or three rungs outperform five in most catalogues.
Measure revenue per visitor, not average order value, because AOV rises whenever a threshold exists regardless of whether you earned anything.
Slightly above the point where most of your orders currently land, so a meaningful share of shoppers are close enough for one more item to be a reasonable step.
The full guide has the formats, the deadlines and the worked examples.