Let the additional fee , the shipping company charges be $y .
So for 0.75 pounds, since the weight is less then 1 pound, company charge = $2x
For 3 packages with weigh 2.5 pounds, company charge 3* ( x+(2.5-1)*y )
= 3x+4.5y
For package with weighs 4.2 pounds, company charge x+(4.2-1)y = x+3.1y
So the total amount , company charge = 2x +3x +4.5y +x+3.1y
=6x +7.6y