Answer:
The cost of the small package
Step-by-step explanation:
Cost of 1 postcards = $0.25
Cost of 4 postcards =
Cost of 1 letter = $0.44
Cost of 5 letters =
Cost of 1 large envelope = $0.80
Cost of 2 large envelopes = 
Cost of 1 small package =$5.10
Since the cost of small package is highest
So, It will increase the mean
So, the cost of the small package will skew the mean