Potential function for stack problem
A reasonable potential function here would be $C\ell$ or $C(k-\ell)$, where $\ell$ is the number of elements on the stack.

However, I don't think that the amortized cost can be bounded. Consider $k$ push operations followed by $M$ copy operations. The total cost is $k(M+1)$. If the amortized cost of push is $a$ and of copy is $b$, then $ka + Mb \geq k(M+1)$. Dividing by $M$ and letting $M\to\infty$, we obtain $b \geq k$.