Are there encryption systems that are both homomorphic and commutative?
$\newcommand{\Enc}{\operatorname{Enc}}$
Actually, Pohlig-Hellman exhibits a homomorphism and commutativity.

For the commutativity, see poncho's answer on your other question.

The homomorphism goes as follows
(it is really just plain multiplication, not unlike what is possible with textbook RSA):
$$\Enc(m_1)\cdot\Enc(m_2)=m_1^a\cdot m_2^a=(m_1m_2)^a=\Enc(m_1m_2)\pmod p$$