Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in Hong Kong.
Link: Banggood product page
Coupon (521.99 USD): BGCITYADS2019
Coupon (539.39 USD): od7off
Coupon (570.99 USD): 7bcdf9
(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): BGASZF128, aBGASZF6128, BGASZF6128, affiliate6, BGZF6128, aff7off, BGaDmITAd, BGBFAFF2019
(these coupons are old, so probably won’t work)
Price history and price alert
Check out price history chart (for all/most countries) for ASUS Zenfone 6 6/128GB Global.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).