Ace Industrial Supply / Charged $400 for &free& items

For starters, we have asked their salesmen to stop calling our house because we have found better prices elsewhere on building supplies. They must be desperate for business because they continue to call. During one phone call, the salesman said he had a lot of odds 'n ends lying around he needed to get rid of, and he would like to send it to us just to get rid of it, paid shipping and no charges whatsoever. Told him fine. A box came with an assortment of items (extension cord, shop light, allen wrench set, etc.) along with an invoice with "$0.00" next to each item. The next month, a bill arrived for $400 for all our "free" items which they insist we owe.