Invisalign Express Question?

Hello everyone. I would appreciate input from anyone that has had the invisalign express procedure done. Yesterday, I went in for a consultation and they gave me a figure of $5500 for the Invisalign Express treatment. Also, my bottom teeth are straight. It's just my top set of teeth that need a little straightening. Therefore, I asked if it was okay that I just do the top set of teeth instead of top and bottom in hopes of getting a cheaper rate. However, they told me that they could not do only the top. For anyone that knows, is it true that you cannot just have the top set of teeth done. That you have to have both top and bottom completed for so called alignment reasons? Also, is $5000 too expensive for the "Express" treatment. I was under the impression that Express would be quite cheaper than the regular treatment. I would appreciate input. Thanks!