Negotiation 101 Mess Up - Possible To Fix?

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So i got offered a job today and i made the classic mistake. They offered x and i asked for y which was the actual ammount i wanted. They gave in right away. Obviously i should have asked for z which is more then y but i did not. Is there any way i can fix this when i go in to do paperwork? The offer is not in writing, and was discussed on 2 cell phones with constant break ups. Is it really stupid of me to try and get a little more at this point, or should i just chalk up the l and move on? Any advice on this would help.

Due to the job being a 45 minute commute compared to the current 20 minute commute i was thinking of asking for a little more due to increased gas costs, but i already mentioned this when i asked for y. Perhaps i can say i didn't factor in mornings with traffic?