Customer not paying the bill balance.

[classyteam]

One of my customers does not pay the bill after we provided the service in Full. We delivered the product. Balance is not paid. about 2K balance. They were supposed to pay on spot. 3 weeks are almost gone. They saying they will not pay.
it is big company though and i am not sure that is the Company not paying or that manager who i deal with is just deciding not to pay. I have full proof of the service delivered to them. Everything is in writing.

What would you do?

Thanks for advice.