But you have confirmed the $5.65/L price is pretty consistent and aside from any other volume purchase discounts it's still the best price for a 0W-20 you can get anywhere.
The discount in buying by the case does seem to be at the discretion of the parts guy you're dealing with.
Without trying hard I got a 10% discount from my local Honda dealer on their 0W-20 and I'm sure I could get more in person.
Since we now know more about these oils than any parts guy at Toyota, Lexus or Honda, there's another negotiation tool if you can befriend the parts guy as rcy has none, i.e., you can offer to provide the VOA and MSDS info plus expert explain in exchange for a decent discount!