Help needed with VCS Collections!!!!

I am in Texas and I had a loan with Check n Go online. I know they are legal. After I was not able to pay in full, I went into default, and I set up payment arrangements back in June. The original loan was $1400. I paid back $500 once it defaulted, and then I made several arrangements that I was not able to keep, because I am only paid once a month and they were wanting money on days when I knew I would have none. I finally called them on Sept 7th to make a $100 payment, and I was told they had sent me to a third party the week before. That same day, a lady from VCS Collections called me and I made a $100 payment in good faith, to show her I intended to pay on the account. She said she would settle the remainder for $700, taking off $250 in interest, which should have been taken off anyway. A lady on your forum told me to ask for a DV letter, so I left a message for them telling them I needed that before continuing to pay. I received an irate message stating they were expecting a $300 payment this week, and that I had already acknowledged the debt, that i did not know how their system works! I am afraid to call them back! What do I say? I am the lady who came within hours of having her car picked up for non-payment, and truly, I have to keep my priorities in line. Did Check n Go sell me as a bad debt? I do know that Check n Go has listed me as writing a bad check, and I cannot write a check for groceries! Please continue with your help! What should I do? I am already afraid of these people!