Pre-purchasing products (lumber, rock, etc.) and storing them on your property sounds like a great idea but it will cost you in the end.

If your property isn't large enough your excavation team will need to work around it and continue to move it out of their way. This will slow down their process and add additional machine time to your bill. 

Either store it off site or purchase the product at the time it's needed. Or be okay with the additional charges.