Aliexpress
Click here… to search for SONOFF IP66 Waterproof Box on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. So I have checked for active coupons (all category coupons and old ones), but there was no working coupon in 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): elec, e2bcea, SONOFF15, BG9NY, BGVDay10%off, bGhOnEy10, BGSMOFF, BGSONOFF15, BGSAMRTH, BGELIG0920, BgcLuBofErtA10, BGSONOFF16, BGAFF10OFF, BGSTHO15, BGHONEY2110, 6GSGIT, BGSmart6, MVC8SPRING, 8BGwbg2204, tRaDetRaCKeR, pROMo8cuPoN, BGJUNEGSG9, BG15Sonoff, TTSUMMER8, Cuponation9, 9OFFSUMMER
(sometimes old coupons work, so you can check them out (this site checks them on every price update))
Price history and price alert
Here is price history (for all/most countries) for SONOFF IP66 Waterproof Box.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).