Hi, i am new here so bare with me , i never had a problem in my life

I was approached by a ppi company letting me know that i had been paying to my bank for my account and i was entitled to compensation, i agreed to go for the claim, at the same time lots of company call and send you packages, i never asked if they were about to be paid, i though it was free as they say, suddenly i completed the form on a rush as i had the agent on the phone asking me to sign the form while he was speaking to me, so i didn't have time to read, a few weeks later, i was contacted by the bank and in deed i was refunded 1600 in total for 15 years of been paying. As i was unemployed the money was really handy and i was able to pay the rent and bills, three weeks after i started to be contacted again from different ppi companies, i declined everyone as I already had the claim, but the last call the lady left a message saying to me that they already had done completed the job and they were expecting payment, i then answer to another agency and asked if they charge for the payment, they said they will get the 17% from the premium, now i spoke to the lady and asked how much their fee is they are asking for 39% +VAT, at the moment i cant afford paying anything, as I don't have that money any more, i offered the 17% that the other company were about to charge, but she declined and said that I wont be contacted any more, the next day i received an email saying that as I had refused to pay the claim will be referred to a third company that will be harassing me for payments and to be aware that I will be paying more charges.
Why are they not able to negotiate?
They are really keen into make you sign the papers but they are not clear with the fees.
They are very crude and rude when asking for the payment, there is not understanding whatsoever.
Can I keep offering the 17% (not that I have that money but it is better that 39%

What can i do?