Trucker Knight-Swift Transportation Holdings Inc. is pushing into a crucial transport sector that moves smaller shipments for retail and industrial clients, as red-hot freight demand strains US trucking capacity.The Phoenix-based truckload behemoth is buying regional operator AAA Cooper Transportation Inc. in a transaction with an enterprise value of $1.35 billion, the company stated on Tuesday.The deal closed on July 5 and consisted of $1.3 billion in cash, $10 million in Knight-Swift shares, and roughly $40 million in presumed debt. Dothan, Ala-based AAA Cooper, which serves clients across Southeast and Midwest, will continue to operate independently under the Knight-Swift umbrella.The acquisition gives Knight-Swift, the biggest US truckload carrier, a foothold in the less-than-truckload business, in which trucking companies combine shipments from multiple clients on a single trailer.