We consider a single-item, infinite-horizon, continuous-review (S-1,S) inventory system with Poisson demand and stochastic leadtimes. In the business scenario examined, the supplier exerts a one-time effort that reduces the mean and/or variance of replenishment leadtimes, and the inventory manager of the system makes periodic payments to the supplier based on realized leadtimes. Assuming an exponential utility function for the supplier and a normal leadtime distribution, we provide the parameters of a contract, under which the supplier periodically receives a fixed amount plus the sum of linear incentive payments for the replenishment orders delivered during that period based on their leadtimes. Extensive numerical experiments suggest that the cost impact of the leadtime reduction can be large, and that the proposed fixed-amount-plus-linear-incentive contract is very effective.