price + fraction of a price

the cost to overnight air something with a weight of 0-2lbs is: $18 dollars

however, for the weight, for each pound or fraction of a pound above 2 lbs, is a +$6.00

so whats going on?

i understand that for something that is 3 pounds. It would be $18 +$6.00 correct?

so what if its 5 pounds, or 6.6 pounds?

would .6 be the same as 1 pound?

i'm confused if there is for example: .2, .4, .9, or .1 pounds.. how much extra would it cost? would it automatically just cost $6.00 for any of those extra weights above 2 lbs.

thanks.