For $f$ and $g$ polynomials in $p$ variables, we relate the special value at
a non-positive integer $s=-N$, obtained by analytic continuation of the
Dirichlet series $$ \zeta(s;f,g)=\sum_{k_1=0}^\infty ... \sum_{k_p=0}^\infty
g(k_1,...,k_p)f(k_1,...,k_p)^{-s}\ \,(\re(s)\gg0), $$ to special values of zeta
integrals $$ Z(s;f,g)=\int_{x\in[0,\infty)^p} g(x)f(x)^{-s}\,dx \, \
(\re(s)\gg0).$$ We prove a simple relation between $\zeta(-N;f,g)$ and
$Z(-N;f_a,g_a)$, where for $a\in\C ^p,\ f_a(x)$ is the shifted polynomial
$f_a(x)=f(a+x)$.

By direct calculation we prove the product rule for zeta integrals at $s=0$,
$ \mathrm{degree}(fh)\cdot Z(0;fh,g)=\mathrm{degree}(f)\cdot
Z(0;f,g)+\mathrm{degree}(h)\cdot Z(0;h,g), $ and deduce the corresponding rule
for Dirichlet series at $s=0$, $
\mathrm{degree}(fh)\cdot\zeta(0;fh,g)=\mathrm{degree}(f)
\cdot\zeta(0;f,g)+\mathrm{degree}(h)\cdot\zeta(0;h,g). $

This last formula generalizes work of Shintani and Chen-Eie.