To find out how much Mrs. Perez should write the check for, we need to subtract the amount she paid in cash from the total cost of the refrigerator.
Total cost of the refrigerator: $1368.25
Amount paid in cash: $497.80
Now, we perform the subtraction:
\[ 1368.25 - 497.80 = 870.45 \]
So, Mrs. Perez should write the check for $870.45.