This paper studies a sustainable last-mile delivery problem, which is one of the most complex vehicle routing problems. The problem consists of customer parcel allocation and the routing of an electric vehicle and a heterogeneous fleet of drones to distribute parcels to customers. To optimize this problem, a Mixed-Integer Linear Programming (MILP) model is formulated with the objective of minimizing the total weighted tardiness. The model is solved using an exact approach with the CPLEX optimization solver on nine realistic problem instances ranging from 5 to 15 customers. Numerical results show that parcel delivery planning with multiple heterogeneous drones and a single EV provides better solutions than single drone-EV combinations.