Aliexpress
Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China. There is no active coupon in my database right now (for Banggood).
Link: Banggood product page
Price: -3 USD
It looks like this item is out of stock in Banggood. If you found it then please send me the link. I will replace it.
Click here to search on Banggood!
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): BGRCRR9, BGCuponation10, BGRRCB10, BGf5ee43, BGb530fc, BG5fabcc, BG8e124d, Parts8, 00c906, BG00c906, ADMITAD2212, 2022ACTIESNL, Tabby20, BGRCParts8, BG1efeb0, 8Couponsde, BGCAPITALOS8, BGCOUPERT8, BG8FORMULAMX, BGBFDAY8, BGNEWYEAR8, BGSPRING8OFF, BGHONEYSP
(these coupons are old, so probably won’t work)
Price history and price alert
Here is price history (for all/most countries) for FrSky Tandem X20 RC Transmitter.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).