You agreed to $419 when you purchased the items, the price the day before, the day after or any other day is irrelevent to your agreement. Anyway, Bitmain offering coupons - which are a gift and not a requirement - are as good as cash. You can sell those coupons to others if you don't want to use them yourself.
Exactly the attitude that makes BTC appear supremely suspect as a medium of commerce. The recipient clearly has no responsibility to provide equitable value! It was Bitmain that recognized the correct market price was actually $369, I'm part of that market! Further, the coupon can only be sold at a significant discount, so basically, when buying from Bitmain, it's a lottery on the price, and expect limited value for any compensation you can't make use of. Guess I was wrong Dogie, you are purely a lackey of Bitmain!