What is your travel day as that will affect the first question of is it OK to wait to buy when you get there? Normally, it is OK except at the beginning and end of long holiday weekends or the important travel crush times during Argentina and Chile summer vacations. Walk-up ticket prices can rise and fall depending on demand.

Santiago-Valpo route is done by multiple companies including the two biggest bus companies Tur-Bus and Pullman (Costa division). Tur-Bus you can buy direct at the cheapest price from their website but again only with PayPal as a foreigner. Pullman Costa website does not have any purchase options for a foreigner though you can buy a ticket at a higher price via the recorrido site (again with PayPal).