That's the way these places work. They demand payment in full immediately - this scares you and makes you see an urgency to their demand. They assume you will be so scared that if you do have a means of coming up with the money, you will rush out to get it and send it to them to avoid "the next step." The reason you want to demand validation at this point is because it gives you a starting point. 1. You make sure that the company you are paying actually DOES own the debt (it's possible you can receive a letter and the debt is sold to another company by the time you send a letter or payment). 2. You get a breakdown of the amount owed to the original creditor and the interest and other fees being added on by the CA. 3. You get an exact date of last activity or default.
Having the amount owed to the original creditor is where you want to start your negotiations. You CAN get the CA fees and additional interest waived, they are being paid out of the original amount owed - anything else they can get is gravy. During the validation process, you can also spend that time reviewing your income and expenses and figuring out just what you really can afford to pay. Then, once the debt is validated, lay out your proposal to the CA in writing. Write to them that considering your expenses and other bills $XXX is what you can afford to pay them on a monthly, semi-monthly, weekly basis. Stay in control and know what you can and can't do. When you send your offer letter, also send the first payment to show good faith. If they accept and cash that first check, then that can be interpreted in a court of law as acceptance of your terms. Any further harassment on their part can be deemed a breach of contract. Then stick to the agreement you proposed - don't miss any payments.