Disputed invoice with a vendor - vendor was under a monthly retainer (always paid in full and on time). Several months in, the vendor sent me an invoice equal to 4 times the value of the retainer - for "setup" costs. These were for services covered under the retainer; however, the vendor felt he did more work than he anticipated and deserved extra compensation. I received no notification of any of this prior to receiving the invoice.
The agreement for the retainer was verbal. I do have notes from the meeting showing the cost and services under the retainer, as well as no indication of any setup costs.
Have tried to settle with no luck - now he is threatening to go to collections. I feel this may be extortion, but will cost more to fight than pay, and will hurt my credit. What can I do?