To determine the total cost for an order of \( x \) ink cartridges, we need to take into account the cost of the cartridges and the flat shipping fee.
The cost of the ink cartridges is given by: \[ 19x \] where \( 19 \) is the cost of each cartridge and \( x \) is the number of cartridges purchased.
The flat shipping fee is \( 7.50 \).
Therefore, the algebraic expression for the total cost \( C \) for an order of \( x \) cartridges can be written as: \[ C = 19x + 7.50 \]