Compare fee models (per-order, per-line, storage), SLA guarantees for transit and pick accuracy, and expected volume tiers. Model cost-per-order across seasonality scenarios, require standard KPIs (OTIF, pick accuracy, TAT), and validate chargeback clauses and peak surcharges before committing.

Request data on dock door count, pallet positions, racking density, cross-dock capacity, cold-chain availability, and automation level. Verify carrier integrations, on-site carrier access, typical transit times to major markets, and the 3PL's parcel and LTL lane coverage maps.

Location affects transport miles and last-mile costs; labor rates and productivity drive picking and packing cost-per-order; higher rents increase storage component of cost-per-SKU. Model total landed fulfillment cost combining storage, handling, and inbound/outbound transport for accurate comparisons.

Insist on WMS APIs or EDI connectivity for orders and inventory, real-time inventory feeds, cycle-count reporting, exception alerts, and a dashboard with KPI visibility. Confirm SLAs for data latency, reconciliation cadence, and support for custom reporting and EDI message sets.