Please note: This feature is currently in beta and is only available to selected new users as part of an A/B test.
When using quantity-based shipping cost tiers, some order quantities may not exactly match the tiers you configured. For example, you might define shipping costs for 4 items and 6 items, but an order could contain 5 items or 9 items.
In these cases, TrueProfit estimates the shipping cost using linear calculation methods based on the closest tiers. This ensures the system can still produce a reasonable shipping cost even when the quantity does not exactly match a configured tier.
There are two situations:
Linear interpolation – when the quantity falls between two tiers
Linear extrapolation – when the quantity exceeds the highest tier
1. Linear interpolation (when quantity is between two tiers)
Linear interpolation is used when the order quantity falls between two configured tiers.
TrueProfit calculates the shipping cost by estimating the value between the two known tier points.
Example
Configured tiers:
4 items → $24.43
6 items → $30.31
Now suppose we need to calculate the shipping cost for 5 items.
Step 1: Calculate the cost increase per additional item: (30.31−24.43)÷(6−4)=2.94
This means the shipping cost increases by $2.94 per additional item between these tiers.
Step 2: Apply that increase to the lower tier: 24.43+(5−4)×2.94 =27.37
Shipping cost for 5 items = $27.37
Simple way to think about it: 5 items sits between 4 and 6 items, so the system takes the two known shipping costs and fills in the missing value between them.
2. Linear extrapolation (when quantity exceeds the largest tier)
Linear extrapolation is used when the order quantity is higher than the largest tier you configured.
Instead of stopping at the last tier, TrueProfit extends the cost trend from the last two tiers to estimate the shipping cost for larger quantities.
Example
Configured tiers:
4 items → $24.43
6 items → $30.31
Now suppose a customer orders 9 items, which is higher than the maximum configured tier (6).
Step 1: Calculate the rate from the last two tiers: (30.31−24.43)÷(6−4)=2.94
This means the shipping cost increases by $2.94 per additional item in the latest tier range.
Step 2: Extend that rate beyond the last tier: cost(9)=30.31+(9−6)×2.94 =39.13
Shipping cost for 9 items = $39.13
Simple way to think about it: The you only configured shipping costs up to 6 items.
If a customer buys more than that, the system assumes the cost continues increasing at the same recent rate, which in this example is +$2.94 per additional item.
This allows TrueProfit to estimate shipping costs even when the order quantity exceeds the configured tiers.