1x English User Manual, Package Contents: 1x Relay Tester.
You can create price alert here, copy coupons and view price history. Best price?
Aliexpress
There’re some similar (or not) items on Aliexpress
You can use link bot to get even lower price:
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.
Link: Banggood product page
1. ($15.96): BGAutos
2. ($16.15): BgAuMotor
coupon code has to be applied in shop’s cart 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): TTxBG10, AM15prime, AMPrime01, Automo, Automo1
(you can check these old coupons out, but these probably won’t work)
Price history and price alert
Here is price history (for all/most countries) for AE100 12V Relay Tester. The lowest price was: $11.09.
Price records source(s): Gearbest, 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)).