Use a piecewise linear model
A courier changes its per-parcel rate after a stated number of parcels
Let \(x\) represent parcel count (parcels), and let \(y\) represent delivery charge (dollars). The model is valid from 0 to 7 parcels
The piecewise model is:
\[y = 26 + x,\quad 0 \le x \le 4\]
\[y = 22 + 2x,\quad 4 < x \le 7\]