score:5
I went to Expats and Locals Living in Sihanoukville Facebook page and asked this same question so far I got two names that people swear by.
Ana travel - I called them and they do speak 'understandable English' their rate is 5$ + visa cost, turnaround time is one week.. Their rating is really bad on Google Maps (2.8), but everyone on Facebook seems to trust them and blame some fake impostor company.
Mottah Travel - Second agency recommended by a few.
You need to bring your passport and money to the agency.