Bus service from Santiago to Valparaiso runs every day usually from early in the morning to 10:30PM in the evening every 15-20 minutes. There are multiple bus companies (3~4) serving this route including Pullman and Turbus.
CLP 6,800 is the average price for a ticket. If you book last-minute (which I do frequently), you can save around 800 pesos or so. There is absolutely no need to book in advance, usually you can walk up and book a ticket for the next bus (around 10 – 30 minute wait). I’ve never paid with credit card, but it shouldn’t be an issue.
In terms of the issues you’re getting on Pullman.cl you may need to specify with terminal you are leaving from, most Santiago-Valpariaso buses leave from Pajaritos. Also try turbus.cl I find their website easier to navigate.