Exclusively for Prime members, Amazon offers preorders of ARMS for Nintendo Switch for $47.99 with free shipping. That's the lowest price we could find by $11.
Note: It's scheduled to be released on June 16.
Just an FYI: Prime members get most new released with a $12 discount off the price. They also offer the pre-order price for the entire week -- so if you are not sure about a game, you can wait for reviews to drop before ordering and not miss out on the deal.