I’ve seen companies run advertisements for a drawing, then never do the drawing.
I’ve seen companies advertise for a product that was never finished.
The simplest way to keep your marketing promises is to fulfill them before you market. If doing a drawing, buy all the prizes before announcing the drawing. Finish the product before running the first ads.
Keeping promises is a habit, after becoming better at it, then you can start running the ads in front of the product release.