There are 17 words that start with 'posts'.
| posts | postshow | 
| postsync | postsyncs | 
| postscript | postseason | 
| poststrike | postsynced | 
| postscripts | postseasons | 
| postsyncing | poststimulus | 
| postsurgical | postsynaptic | 
| postsecondary | poststimulation | 
| poststimulatory | |
Note: These 17 words that start with 'posts' are found from the USA & Canada version of the tournament Scrabble word list.