19. A large supermarket holds a promotional event during the New Year's Day holiday. It is stipulated that no discount is given for a single purchase not exceeding 100 yuan, a 10% discount is applied for a single purchase exceeding 100 yuan but not exceeding 300 yuan, and for purchases exceeding 300 yuan, the first 300 yuan is still discounted by 10%, while the excess amount is discounted by 20%. Xiao Mei made two purchases, spending 94.5 yuan and 282.8 yuan respectively. Now, Xiao Li decides to buy the same items that Xiao Mei bought in two separate purchases, but in one go. How much should Xiao Li pay?
$$
\begin{array}{l}
\text { Three, 19. Note that } \\
100 \times 0.9=90<94.5<100, \\
300 \times 0.9=270<282.8 .
\end{array}
$$

Let the original price of Xiao Mei's second purchase be $x$ yuan. Then
$$
(x-300) \times 0.8+300 \times 0.9=282.8 \text {. }
$$

Solving for $x$ gives $x=316$.
Below, we discuss two scenarios:
（1）If Xiao Mei's first purchase did not receive a discount, and the second purchase's original price exceeded 300 yuan, then Xiao Li should pay
$$
\begin{array}{l}
(316+94.5-300) \times 0.8+300 \times 0.9 \\
=358.4 \text { (yuan) } .
\end{array}
$$
（2）If Xiao Mei's first purchase's original price exceeded 100 yuan, and the second purchase's original price exceeded 300 yuan, then the original price of the first purchase is
$$
\begin{array}{l}
94.5 \div 0.9=105(\text { yuan }) . \\
\text { Therefore, Xiao Li should pay } \\
(316+105-300) \times 0.8+300 \times 0.9 \\
=366.8 \text { (yuan). }
\end{array}
$$