It's no secret that kids can get bored with something if they don't see how it will benefit them directly. If you're trying to keep them motivated about maximizing their school fundraiser sales, you need to translate the fundraiser in to terms they will understand. "This will benefit everyone" is probably not good enough to keep them excited and keep their school fundraiser sales high. You need to be more specific: "Your personal school fundraiser sales will pay for our trip at the end of the year," or "you want those cool new uniforms, right? So lets get out there and get excited about school fundraiser sales."
You have to strike a balance between focusing on school fundraiser sales and focusing on the desired results. Obviously you want the kids to think in terms of all the great things they will have as a result of their school fundraiser sales, but you don't want them to get so caught up in daydreaming about it that they forget to put in the effort required. On the other hand, you don't want to push the sales aspect of your school fundraiser so much that they feel pressured, or that school fundraiser sales becomes a chore for them. So keep it light, keep it fun, but do remind them that their school fundraiser sales will benefit them in very direct ways, and you will see motivation from them like never before.