Multi-carrier shipping is the practice of routing each parcel to whichever carrier delivers the best combination of price and service for that specific shipment — rather than committing every parcel to a single carrier under a long-term commercial contract.
Most US businesses ship through a single carrier (usually UPS or FedEx) because that's what their original commercial contract gave them. That setup is simple to operate but expensive at scale: every parcel pays the same carrier's rate even when a different carrier would be cheaper or faster for that specific lane.
A true multi-carrier program rate-shops in real time across USPS, UPS, FedEx, and DHL — and assigns each shipment to whichever wins. A 2 lb parcel going coast-to-coast might be cheapest on USPS Ground Advantage. A 15 lb parcel going to a residential address in the same metro might be cheapest on FedEx Home Delivery. A bulk pallet might be cheapest on DHL eCommerce. The savings compound.
The catch is that running multi-carrier well requires contracts with every carrier, integrated rate-shopping software, and someone watching the rate stack as carriers change pricing each year (the General Rate Increase, or GRI). Most businesses don't have the volume or operational bandwidth to do this themselves — which is why they outsource it to a shipping partner like L5 who runs the carrier relationships and rate-shopping for them.