I'm thinking the website is not, nor has been indicative of the complete size offerings of ML. I don't see 5qt/5+qt jug sizes pictured/listed in Valvoline/MaxLife motor oil offerings on their website either, but they are common and available. Afaik and can remember the quart size is all that's ever been pictured as part of their site. In checking local WM's, just noticed ML gallons has very recently been added as a stock item at the closest area WM.
That aside, $13/gal for ML is a good price.