How do I get proof from Dwp that they have received my renewal form

Sent my Pip Renewal form by special Delivery on 03/07/2023 says it was signed for by Dean on 04/07/2023, rang today to check to say if they have received it in the system and was told no not recived but I do have until 17/07/2023 the Deadline date to send it back , I said I already have sent by special delivery recived and signed on the 4th , the advisor said they have no way of checking special delivery track n trace , but will extend my deadline date to give me extra time to send it back , I said I have already sent it back and was delivered and signed on 04/07/2023 again the advisor said all she could do is give me extra time to send it back , any tips ideas what I should do , should I ask for another form ,or should I just keep ringing every day to see if goes into the system , what happens if I do nothing and just sit here with proof my of special delivery do I just use it as an appeal if they stop my money because they said I never retuned it by deadline or extended deadline, Help getting stressed Sharon