The baseline Standard Edition of Borderlands 3 costs $59.99, and that is what you will have to pay upfront to preorder the game. If you are a Prime member, Amazon will email you a $10 promotional credit 30-35 days after the game ships. Yes, it's a bummer having to wait a month, but it beats preordering the game and getting nothing in return.
There's also a second bonus offer. When you preorder Borderlands 3 for the PS4 or Xbox One, you will receive a Gold Weapon Skins Pack. This consist of a gold weapon trinket and gold weapon skins that are applicable to all guns in the game (this is not exclusive to Amazon).
Here are some links to where you can preorder Borderlands 3...
Borderlands 3 released to PC and consoles on September 13, 2019.