I have recently received a handshake offer to work a full time job at a location some hours drive away from where I currently live. A day after I tenatively accepted the job, I was informed by my hiring manager via phone that I would be sent an offer letter via snail mail that would have my salary and benefits on it. He emphasized via phone that the salary on the letter was "negotiable", leading me to believe that it will be a lowball offer.
I did not verbally agree on any salary during the interview. I was offered the job directly after the interview and I gave them a tenative start date for availability (so that they could put that start date on my offer letter), but no salary was agreed upon.
What is the best way to negotiate this amount and request other benefits such as a relocation package? Should I rewrite the employment contract with my requested amounts and send it back to them?
Edit: I'd like to fill in on what happened in this situation. I received the offer (it was the lowball offer I had expected) and negotiated my salary via phone, and followed up with an e-mail as advised here.