(Reuters) - United Parcel Service Inc ( UPS.N ) said on Tuesday it is buying 125 Tesla Inc ( TSLA.O ) all-electric semi-trucks, the largest known order for the big rig so far, as the package delivery company expands its fleet of alternative-fuel vehicles. Tesla is trying to convince the trucking community it can build an affordable electric big rig with the range and cargo capacity to compete with relatively low-cost, time-tested diesel trucks. This is the largest public order of the big rig so far, Tesla said. Tesla unveiled its semi last month and expects the truck to be in production by 2019. The Tesla trucks will cost around $200,000 each for a total order of about $25 million. UPS expects the semi-trucks, the big rigs that haul freight along America's highways, will have a lower total cost of ownership than conventional vehicles, which run at about $120,000. Tesla has received pre-orders from such major companies as Walmart ( WMT.N ), fleet operator J.B. Hunt Tra...