Basically copy coupon, view price history or create e-mail price alert. Let’s see what best price is ATM.
Weight: 45g, this package doesn’t come with the battery., Wireless frequency: 433.92MHz, Operating environment: -10℃-50℃ (14℉-122℉), Battery: length 27mm, Protocol: EV1527, Quiescent current: ≤5μA, Emission current: ≤10mA, Product Size: 77*49.5*18.8mm, diameter 10mm, Support Battery: 12V 23A
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. There are no active coupons for this product, but the price is pretty low isn’t it?
Link: Banggood product page
Price: $-3
It looks like this item is out of stock in Banggood. If you found it then please send me the link. I will replace it.
Click here to search on Banggood!
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): elec, BGVDay10%off, 0e7f33, FBCOUPON1, bGhOnEy10, BGSMOFF, BGSONOFF15, BGSONOFF16, BGSTHO15, 13f142, BGHONEY2110, BGSHOM15, BGSHE15, 6GSGIT
(there’s a little possibility that old coupon will work)
Other warehouses/versions
This item is available in 1 another warehouse/version!
1. Europe – 5Pcs SONOFF DW1 433Mhz Door Window Sensor [EU/UK] ($-3)
Price history and price alert
Price history (for all/most countries) for 5Pcs SONOFF DW1 433Mhz Door Window Sensor is here.
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)).