I just preordered a Lilac Purple S9 from Verizon.
I had been debating whether to order from Verizon, Samsung, or Best Buy. What I found out by going thru the ordering process for each online was that all three options required you to use the 24 month payment plan in order to trade in a phone and get the discount.
So for me Verizon was $599.99 (with a $200 trade in), Samsung was$649.99 (with a $150 trade in) and Best Buy was $691.92 with the $100 off for March 2 orders only.
With Best Buy I didn't see how you could apply a trade in gift card that would go towards to cost of the phone since you had to accept the 24 month plan or else pay the full price which was $792. If I had bought it outright from BB then the cost would have been $642 IF I could apply the gift card at the time of purchase. And that is iffy. Some have said yes you can and others have said no. At any rate that still came out to $50 more for me over Verizon.
You do have to pay for sales tax for the full price and a $30 activation fee.