Making the stripe and weeding it out backwards gives you the stencil. They will likely charge you for the stripe as that is essentially the amount of vinyl your buying. Easier to buy the stripe and pull measurements from it and then sell it get some of your money back.

Other options would be to see if they can plot a line drawing on paper of the stripe instead of cutting it out of vinyl, or go to a car show or junkyard and pull a pattern off another car.