I'm also not sure what that discrepancy could reflect. Like WisNeuro mentioned, the annual COLA (when it happens) doesn't occur until January. A 12% difference would equate to about ~4 step increases. Does the amount they quoted match any of the values in the salary table exactly? Perhaps they either sent you the wrong table or quoted the wrong amount in the letter (e.g., they have you listed coming in as GS13 step 3, but wrote in your letter the salary for a GS13 step 7 for some reason).
I hate to say it, as there's a chance it could be a mistake and they'll correct downward, but I'd probably ask.
As far as negotiating, the office supplies and such will generally be provided on-site. I will say even if you don't get something nice to begin with, you may have the opportunity to order new furniture at some point in the future. That being said, the likelihood of you getting standard office equipment (e.g., chairs, desks, etc.) that are different from your colleagues is slim unless there's a medical reason for it. It might be some kind of union issue. You may be able to request something like a standing desk, but you could just as easily do that once you're an employee.
I did negotiate a bit with the testing supplies they purchased when I arrived. Or rather, I requested X amount, they said, "we can give you Y, tell us what you need ordered," and that was it.
If it's a hard to fill position, you may be able to request EDRP. But if you already have the offer letter, they may be hesitant to go back and make changes now. Good news is EDRP can become available down the line in some situations.
Things like relocation bonus/moving expenses would've been listed in the job ad. Step increases based on past experience are generally pretty standardized by HR at each site (although that's not at all to say they're the same nationwide).
So all in all, it's a blessing and a curse--there's not much you can negotiate, but you don't have to worry about having missed out on much if your negotiation skills aren't great.
