To find the rate of change, we can calculate the difference in the total cost for each additional pizza ordered.
For the first two pizzas, the total cost increased from $19.98 to $29.97. The difference is $29.97 - $19.98 = $9.99.
For the next two pizzas, the total cost increased from $29.97 to $39.96. The difference is $39.96 - $29.97 = $9.99.
Therefore, the rate of change in the total cost for each additional pizza ordered is $9.99.