It's indeed a huge flaw, but he first did a study of the market in the city to get to his idea of delivery.
He indeed could have asked the potion shopkeeper about laws and such.
I think that the level of Susanoo didn't "pass on" to MC hence that's why Orochi wasn't killed in 1 shot
(Only my assumption, I don't know if the level of a card being passed on to MC was detailed in earlier chapters or just wasn't explained)