QBKG21LM.
Just check coupons out (with price alert and history). What is the best price?
Aliexpress
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Click here to search by yourself. Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China. We have found one coupon for this product!
Link: Banggood product page
1. ($28.89): BGSmart6
paste coupon code in your cart and click Apply after clicking “Proceed to checkout”. Click to copy code and open shop.
Additional Banggood coupons: first page and second page
Old coupons (from the oldest to the newest): BGAQAD1S, BgcLuBofErtA10, BG15SH12, BGSTHO15, 13f142, BGHONEY2110, BGSHOM15, BGSHE15, BG0859c9, BGe02387, BGf5d5cc, BGAQA929, BGd324bf, BG10c204, BGD11055, BGB5067, BG479f9e, BG06aabb, ADMITAD2212
(system checks these coupons on every price check (like once per 24 hours), but you can check them out anyway)
Other warehouses/versions
This item is available in 1 another warehouse/version!
1. Europe – Aqara D1 QBKG24LM Zigbee Smart WIFI Wall Switch [EU] ($-3)
Price history and price alert
Here is price history (for all/most countries) for Aqara D1 QBKG21LM Live Line Zigbee Smart WIFI Wall Switch. The lowest price was: $10.5.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).