An outsourced partner that stores your inventory, picks and packs orders, and ships them to customers. A good 3PL lets you scale fulfilment without building a warehouse; a bad one becomes the bottleneck that throttles growth and trashes your customer experience. The handover from self-fulfilment to a 3PL is one of the highest-leverage operational decisions you make.
Benchmark. Judge a 3PL on shipping accuracy, dispatch speed, and error rate, not just per-order cost. A cheap 3PL that mis-ships erodes retention and support load faster than it saves you.