Extra money

Today I finished a project I was doing for a company in which I fabricated and welded four carts. The company will use the carts to load and unload inventory. If they find the carts efficient for their needs then they will contract me to make another 6 or 16. Hopefully they want more carts since I enjoyed building them. If they do, then I would make about $2500 altogether for building 20 carts. Money that I'd like to put into some Treasury notes or such.