It's common knowledge by now that the Galaxy S6 and the Galaxy S6 edge don't have a removable battery so it's not possible to swap out an empty battery with a fully charged one, as was possible on previous flagships from the Galaxy S series. The battery is actually glued to the metal frame and buried underneath components so a fair bit of skill is required to open the handset and retrieve it. You'll be better off by getting it done through a certified technician when the need arises and now you know that it's going to cost you $45.
A Samsung spokesperson told PCMag that the Galaxy S6 and S6 battery has a one-year warranty so the company will replace it for free if the maximum capacity falls below 80 percent of its initial level during the first year. However before Samsung extends that courtesy the battery's capacity will have to be verified at a service center. At any other time it will cost customers $45 plus shipping to replace the battery, Samsung's walk-in repair center in Plano, Texas can get it done for customers on the same day they bring their device in. The company is looking to establish more of these repair centers across the United States.
This is how much it will cost customers in the US to get the battery on their Galaxy S6 and S6 edge replaced. Samsung likely has similar pricing for replacements in other markets around the globe, though it will depend on the market itself whether the company operates repair centers that offer same-day swaps or not.