An invoice for a shipment is shown below.
The delivery fee is calculated as a percentage of the cost of the order. The grand total is the sum of the cost of the order and the delivery fee.
$$ \text{Invoice Details} $$
$$ \begin{array} {c|c}
20 & \text{Item \#3892} \\
15 & \text{Item \#2215} \\
5 & \text{Item \#3100} \\
\end{array}
$$
$$
\begin{aligned}
\text{Delivery Fee: }& \$25.28 \\
\text{Grand Total: }& \$193.78
\end{aligned}
$$
What percentage of the cost of the order was used to calculate the delivery fee?
We first calculate the order total.
$$ \text{Grand Total}-\text{ Delivery Fee}=\text{Order Total}$$
$$ 193.78-25.28=168.50$$
The percentage of the delivery fee of the total can be calculated:
$$ \% \text{ delivery fee} = \frac{25.28}{168.50} $$
$$ =0.15 = \boxed{15 \%} $$