Let \( x \) be the cost of a container of milk. Then the cost of a sandwich is \( x + 0.75 \).
The customer buys 4 sandwiches and 4 containers of milk, so the total cost can be expressed as:
\[ 4(x + 0.75) + 4x = 7.00 \]
Expanding this gives:
\[ 4x + 3.00 + 4x = 7.00 \]
Combining like terms results in:
\[ 8x + 3.00 = 7.00 \]
Next, we subtract \( 3.00 \) from both sides:
\[ 8x = 4.00 \]
Now, divide by \( 8 \):
\[ x = 0.50 \]
The cost of a container of milk is \( 0.50 \). Therefore, the cost of a sandwich is:
\[ x + 0.75 = 0.50 + 0.75 = 1.25 \]
Thus, the cost of a sandwich is:
\[ \boxed{1.25} \]