While there are lots of useful posts on this with links to bus companies, having looked online at most of these operators, they all only seem to offer evening bus options > 16:30 from their websites, yet I see talk of people having got the bus during the day.
So I guess my questions are:
1) Do we need to book our bus online as soon as possible
2) Do companies such as AndesMar offer buses from Santiago to Mendoza earlier than their 16:30 option on their websites?
I've tried looking ahead of time to see if it's because they've sold out the earlier tickets, but this appears to be the earliest they offer.