Allowing downpayment Is the a way to allow downpayment for customer-preoders.
I can create a content type that allows customers to place orders(pre-order) but I wish they could commit a certain fee, like 30% of the original price.
Is there a way to achieve this?
There are plenty of modules are available for managing inventory and orders. Like Ubercart, Commerce. 

But according to your requirement it seems that you want a partial payment system. 

The default modules available in drupal community will not be able to provide you such kind of feature.

My Suggestion is to create it completely using your custom module and manage the complete order and partial payment process system.

Hope this will help you.