One day only sale at Sam's Club. Buy a Samsung Galaxy S8 or S8+ on installment at Sam's Club on August 5 and get a $350 Sam's Club Gift Card + free activation. Price of phone varies my model and carrier. If you don't value the Sam's Club GC, you should be able to buy a Visa GC in store for about a $5 fee. I found no other Samsung promotions currently on this phone.
Depending on how you value the Sam's GC (to me it is close to cash), it puts the phone at just under $450 with taxes. I see no reason why the phone cannot be paid off in full at anytime since this deal does not rely on promotional credits - perhaps using a Visa gift card converted from the Sam's gift card.