Purchase by mail

Hi, there,

I sent out a check to a company in Seattle to purchase their products.
They are supposed to send me a package in mail. However, I never
recieved it. I then sent multiple emails to ask about the status.
They only replied once, saying that they can send a replacement copy.
However, I still do not receive anything.

The value of the purchase is around $500. How do i proceed with this?

Any suggestion is appreciated!

Shawn