No they won't charge you the difference, they'll give you an unboxed, without manuals, etc N8 or E7 as a substitute for N900 because they would deem your N900 is not repairable. You wouldn't be the first one to be offered N8 or E7 as many before you have had their N900 replaced by N8 or E7 because of various other faults.

Screen facing towards the ground would definitely damaged the screen and most likely digitiser. I do not know what BestBuy is like because I don't live in the stateside but you might want to get your device repaired by someone whom is actually qualified and wouldn't mind you watching them whilst fixing your device. N900 isn't as popular as iPhone but you wouldn't want the internals of your genuine N900 being swapped out for cheaper knock-off clones neither would you want the technician to do a poor job at fixing your beloved device.