@Richard ********
I've never been charged this 300 Baht though I would at least spend a few hours in Cambodia first before returning. These days, I always make a trip out of it and spend several days, but in my border running days (last time was 2014) I didn't have any extra expenses to pay aside from the visa cost on the Cambodian side yet I returned to Thailand the same day and it was at this very border described here.