Buying decisions / Quantity discounts

Is a quantity discount worth the extra inventory?

A bracket discount, a freight-prepaid minimum or a full-pallet price all ask the same thing: buy more than you need now in exchange for a better price. The return depends on how much of the order earns the discount, how long the extra stock sits, and how often an item runs out anyway.

Why a bracket differs from a price increase

When you reach a bracket, the discount applies to the whole order, including the stock you would have bought anyway. That makes a bracket discount worth more than a price increase of the same percentage: the saving on the normal portion of the order costs you no added investment at all.

A worked example

• The line buys $9,500 a month at cost and orders monthly.
• The supplier takes 3% off orders of three months’ volume, so reaching the bracket means 2 extra months.
• Direct carrying cost is 10.25% a year, the hurdle rate is 20%, and no credit-period benefit is counted.

Extra purchase per order, after the discount

$18,145

Average added investment

$9,073

Annual gross savings, discount on all purchases

$3,420

Return on added investment

27.4%

The return is 27.4%, above the 20% hurdle. A common shortcut, which divides the discount only by the extra months, gives 25.8%. It understates the return because it ignores that the discount also reduces the extra purchase itself.

When fill-in risk reverses the decision

If an item runs out before the next bracket order, you fill in at the normal price. That purchase earns no discount, and the bracket order now lasts longer than planned. Both effects lower the return.

A 30% chance of a fill-in brings the return to 19.5%, just under the hurdle. At 60% it is 13.8%. The same bracket can be a good buy for a stable line and a poor one for a line with erratic staples, which is why the workbench measures that probability from each item’s history.

Freight-prepaid thresholds

A freight threshold is a quantity discount in disguise. Convert the freight charge you avoid into a discount on the order that reaches the threshold, then evaluate it the same way.

Take a line that normally orders $12,000 a month and a supplier that prepays freight on orders of $24,000. If the freight you would otherwise pay is $350 an order, the avoided charge equals a 1.46% discount and the return on the extra month is 25.8%. If the charge is $150, it is a 0.63% discount and the return is only 4.9%. Free freight is not automatically worth an extra month of stock.

Count only freight you would actually pay. If your normal order already ships prepaid, a higher threshold gains nothing.

Details that change the answer

• Start from the discount you already earn. If the normal order already reaches a lower bracket, only the improvement over that baseline counts. Comparing the new bracket against list price overstates the saving.
• Check the order still qualifies after exclusions. Items with no recent demand, less than one unit of extra need, more than a year to sell through, or enough stock already on hand and on order should come out of the buy. The remaining items must still reach the bracket minimum.
• Pack rounding moves the investment. Rounding each item to its pack can push the order over the minimum or add stock you did not plan for.
• A permanent bracket needs more safety stock. If you take the bracket on every order, the order cycle itself gets longer and each item needs more protection between orders. That extra safety stock is added investment too.

Buying ahead of an increase is the other common case. See buying ahead of a supplier price increase.