The short answer
Carrier cost is what the carrier charges the 3PL under its agreement. Customer-billed postage is what the brand pays. They may differ because of percentage markups, flat label fees, rate-card pricing or surcharge treatment. A markup is not inherently bad; compare your actual billed cost and service with realistic alternatives.
Percentage markup vs. flat label fee
Illustrative carrier cost of $6 plus a 15% markup adds $0.9, for $6.9 billed postage. A flat $0.50 label fee instead would produce $6.50. Some agreements use both; others use a rate card rather than cost-plus pricing.
Ask what amount the percentage is applied to: base transportation, transportation plus surcharges or another figure. Markup on cost is not the same as profit margin on billed revenue.
Test a cost-plus example in the Shipping Markup Calculator. It does not account for separately billed label fees or surcharges.
Discounts, rebates and credits
A provider's carrier discounts may still make its billed rates competitive after a markup. Ask what rate basis is used and whether rebates, service credits or refunds flow through to you. A percentage discount from a published tariff is not enough to compare two proposals with different service mixes or surcharge terms.
Do not assume all negotiated carrier terms are available to your own account. Compare the rate you can actually obtain with the amount you will actually be billed.
Surcharges and later adjustments
Fuel and accessorial surcharges, such as applicable residential, delivery-area or handling charges, can materially change postage. Carriers may later adjust a bill after measuring weight or dimensions or correcting shipment data. Applicability and rates vary by service and agreement and can change.
Confirm whether the 3PL passes these through at cost, marks them up or bundles them, and when adjustments appear. Ask how disputed charges and resulting credits are handled.
Dimension corrections are easier to assess after reading What is dimensional weight?.
Transparency questions to ask
- What is the billed-postage basis, and what sample invoice or rate card can I review?
- Is there a percentage markup, flat label fee or both, and to which charges do they apply?
- How are discounts, rebates, refunds and carrier credits treated?
- Which surcharges and later adjustments are separate, and how can they be disputed?
- Can you price a sample of our real destinations, packed dimensions, weights and services?
Include the resulting postage in How to compare 3PL quotes instead of comparing headline discounts.