Is Pohlig-Hellman Cipher the only option?
If you define $E_a(M) = M \oplus  a$ then the key $ab$ can also be defined as $a \oplus b$ as follows:

$E_b(E_a(M)) = (M \oplus a) \oplus b$

$E_{ab}(M) = M \oplus (a \oplus b)$

However using the same key and/or message more than once in XOR will leak the other one. So I think it's potentially not what you're looking for.

As far as I know, SRA is also a variant, where you get regular RSA where both parties use the same modulus (and prime numbers) and create their own encryption, decryption exponents; and keep it secret. This way you achieve the same commutativity as in PH.