In an interview with Gamespot, Game Director Ion Hazzikostas has confirmed that there are no plans to sell Trader's Tender as a standalone product in the store, but it may be included in certain bundles.
3 weeks ago, we datamined new Trader's Tender shop assets which were added on the Patch 10.1.5 PTR. According to
Gamespot, we learn that Blizzard may offer extra Trader's Tender as part of certain bundles such as expansion deluxe editions or big bundles. The assets were meant to be encrypted, but due to an error they were not.
"The overwhelming majority of Tender every month is earned logging in, claiming it, and doing any of a wide array of activities to claim the rest, and that's not something we're looking to change or subvert or upend." - Ion Hazzikostas