Discrete math problem ends up with seemingly impossibly large number.

Based on TonyK's comment, I think I can now answer correctly.

Basically, since we don't care about the permutations for the 2 slices of pizza, the actual number of combinations of 2 slices is $\frac{17^2-17}{2} + 17 = 153$, times $5$ sodas $= 765$.

Each day after that, Bob can choose any of those possible lunches besides one, so $764$.

Therefore the actual number of possible lunch combinations over 30 days, is $765 * 764^{29}$.

EDIT: The reason for $\frac{17^2-17}{2} + 17 = 153$ is that $17$ of those possible $2$ slice combinations, the ones that are the same kind, are going to be unique; but $17 \cdot 16$ of them, where we're picking $2$ different objects from the same set, are going to be counted twice (once in each possible ordering, AB-BA) for each combination, hence why we divide by $2$.