SUBSET-SUM with binary - unary - "positional" encoding
If an instance of POSITIONAL-SUBSET-SUM has input length $N$, then each integer in the set $\{x_1, x_2, ..., x_n\}$ must satisfy $|x_i| \le N$, and the set itself must have size $n \le N$.  So the input length for the corresponding instance of UNARY-SUBSET-SUM is $O(N^2)$, and it is easy to see that mapping one input format to the other can be done in polynomial time.  Therefore, since UNARY-SUBSET-SUM $\in \text{P}$, we have POSITIONAL-SUBSET-SUM $\in \text{P}$ as well.