Regarding to your email, for Boat and Bike tour will be finish around 16.00 tour end service in Ayutthaya. To get back to Bangkok you can easily take train or join minibus.
Train to Hualumpong around 2.00 hrs.
Join bus around 1.30 hrs.
Or the other option, tour code ABT012
the public transport from Ayutthaya to Bangkok will be finish around 18.00 (join minibus to victory monument) as well as a train to Hualumphong.