Amazon offers Prime members the LEGO Creator Red Rotors Set, model no. 31003, for $8.39 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $6.14.) That's the lowest total price we could find by $10. It builds into a helicopter, propeller plane, or hovercraft.
Orders placed via Prime by December 22 are guaranteed to arrive in time for Christmas. (The deadline for standard shipping is December 19.)