Nittany Mountain Excavating

We have a new construction home in ***. The groundbreaking date was Aug 6th 2018. We paid $32,000 for the sitework. On March 27th 2019, our builder passed along the extra bill to us, which was $3,181 and issued by Nittany Mountain Excavating. They asked us to pay for hauling extra fill dirt from construction site to dumping site. The bill only has a number $3,181 but no breakdown of the cost. Our questions are: (1) Why didn't we get the bill right after the job was done (which was almost 8 months ago)? (2) How do they define what is extra dirt? We were at the job site every single day during the building process, and we didn't see many rocks/dirt from the site. Can they give us a cost breakdown for this bill instead of just showing us a number ($3,181)? How did they get this number? We called this company several times but the person who answered the call said he will not let us know the owner's contact number and declined to explain the bill. We emailed them asking for explanation of the $3,181 number, they replied email but didn't answer our question; they just said I should direct this issue to our builder. But our builder didn't answer our question either, simply asking us to pay for the bill with no explanation provided. Since this bill was issued by Nittany Mountain Excavating, we need this company to explain for the charge. Why do we need to pay $3,181 on top of the $32,000 charge? Thank you.