Just check coupons out (with price alert and history). What is best price?
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 code price check (UTC time):
Output: USB 2.0, 5V / 1A, Only flat top batteries are allowed
Banggood
Banggood warehouse is located in China. So I have checked for active coupons (all category coupons and old ones), but there was no working coupon in Banggood.
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): e2bcea, aff7off, BG3CSUM, BGaDmITAd, bgchinafest, BGBFAFF2019, BGCITYADS2019, 2020CNY, BGVDay10%off, BGAFF7OFF, bGhOnEy10, BGXMI15, BGdragonBoat, BGSS10%SW, BGAFF10OFF, BgoFErTaSchInA10, BgcLuBofErtA10, BGHONEY2110, BANGGOODSD
(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
Check out price history chart (for all/most countries) for 18650 Power Bank DIY Case.
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)).