Output: USB 2.0, 5V / 1A, Only flat top batteries are allowed.
Basically copy coupon, view price history or create e-mail price alert. What is 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 and price update (UTC):
Banggood
Banggood warehouse is located in China. I couldn’t find any coupon for this product on latest product update.
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!
Additional Banggood coupons: first page and second page
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
(there’s a little possibility that old coupon will work)
Price history and price alert
Here is price history (for all/most countries) for 18650 Power Bank DIY Case. The lowest price was: $0.99.
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)).