In our case, the Bus from Butterworth to Hat Yai was a white minibus which does not depart from Penang Sentral, but opposite the Pertron gas station. You have to pay 10 additional Ringit per person for the vehicle insurance at the border. Additional note to international travelers: the border guards ask you about your travel itinerary, your bookings for leaving the country and hotel reservations. They also wanted to see a bank account statement or cash. So might want to prepare these documents.
Autobus Express, Billion Stars Express, 11 sie 2022