Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (16.06 USD): Cuponation9
Coupon (16.24 USD): TTSUMMER8
Coupon (16.59 USD): BG00c906
(CouponsFromChina.com tip: coupon code has to be applied in shop’s 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): BGd6f4a5, BG20offcspro, BGAlerabat, 2022ACTIESNL, BGcaseflim, BGJUNEGSG9
(system checks these coupons on every price check (like once per 24 hours), but you can check them out anyway)
Price history and price alert
Price history (for all/most countries) for Nillkin OnePlus 9 Pro Case Bumper with Lens Cover is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).