Valudor Products No payment of past due charges and collection fees based on signed agreement San Diego CA  California

Valudor incurred $3,800 in charges for two transportation shipments.  We have a signed agreement with them stipulating payment within 30 days.  They have consistently paid slow.  After this going on too long, we finally turned these two invoices over to our collection agency which added fees bringing the total to $5,066.66.  The credit agreement that Valudor signed stipulates that they agree to pay these charges.  They initially disputed receiving one of the invoices, but we provided information about when we billed it and emails where we communicated with them about these invoices, so they were aware of them and discussed them.  According to our collection agency, they have stated that they do not care what they sigend, that they are not paying the collection fees and are now demanding we waive the fees to even get paid the original invoices.  This appears very dishonest and unethical to me that they would sign an agreement with payment terms and consequences for not paying timely, and then dishonor the agreement and allegedly state that they don't care about the agreement.