I filed my I-751 packet to early.
I filed before 7/30 and included a $275 money order. Needless to say it was returned to me with a note to the place where I got the money order from, saying that this money order has not been cashed....

I just tried to refund the $275 but they said since I made it out to a 3rd party they will not be able to refund it. They gave me papers to file with MONEY GRAM... guess what that will cost me $12. Its not so much about the $12 its about it might take up to 65 days...

So what I need to know - Can I send the original $275 money order back to USCIS and include another money order for the remaining $270?

You should be able to use that money order in addition to the new one.

My guess, and it's just a guess, is that to resend original money order along with a second money order for the balance due!
Good luck ..........