Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (16.99 USD): 8wEbgaInS5
Coupon (16.99 USD): pROMo8cuPoN
(CouponsFromChina.com tip: apply code in your basket (cart) after clicking “Proceed to checkout”. Click to copy code and open shop)
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): elec, e2bcea, BGBFAFF2019, BGCITYADS2019, BGE13, 13f142, BGVDay10%off, bGhOnEy10, BGTT10, BGSMOFF, bgshOOp10, BGSAMRTH, BGELIG0920, BgcLuBofErtA10, BG15SH12, BGAFF10OFF, BGSTHO15, BGHONEY2110, BGSHOM15, BGSHE15, 6GSGIT, BGSmart6, MVC8SPRING, BG15Sonoff
(there’s a little possibility that old coupon will work)
Price history and price alert
Price history (for all/most countries) for SONOFF RF 7A 1500W WIFI Switch with Remote Controller is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).