Printer ink cartridges cost $19 each. The company that sells the ink cartridges charges a flat rate of $7.50, no matter how many ink cartridges you buy.

The total cost for a single order of \( x \) cartridges can be calculated by considering both the cost of the ink cartridges and the flat shipping rate.

1. The cost for \( x \) cartridges is \( 19x \) (since each cartridge costs $19).
2. The shipping cost is a flat rate of $7.50.

Putting this together, the algebraic expression for the total cost \( C \) of an order of \( x \) cartridges is:

\[
C = 19x + 7.50
\]