Purchase Orders: Tiered Costs Now Read The Right Tier

Product cost by quantity was treating a tier's quantity as an upper bound; it's actually the minimum quantity that unlocks the tier. A 100-unit PO against a ≥ 50 → $X tier was reading the wrong cost. It now matches the price side: the highest tier whose minimum is met wins, ordering zero returns the base cost, and same-day tiers apply immediately instead of waiting until midnight.