Just check coupons out (with price alert and history). What is the 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 price update (UTC time):
Charging Port: Type-c, Product Size: 103x 83x 29mm, USB Output: 5V MAX 2.4A, Material: Metal Case, Welding Output: 4.2V MAX 650A, Product Weight: 302g, Battery Capacity: 5500mAh, Package size: 143x 107x 48mm, Package weight: 407g, Name: Spot Welding Machine
Banggood
Banggood warehouse is located in China. I can’t find any active coupons for this product in Banggood.
Link: Banggood product page
Price: $-3
It looks like this item is out of stock in Banggood. If you found it then please send me the link. I will replace it.
Click here to search on Banggood!
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): BG464337, BGWELDTOOL, BGb4e399, BGANI022, BG8a7644, BG4090b7, BG8dc2f7, CityadsTool, BG3fd745, BGGines, BGPTL088, BG6d2f05
(these coupons are old, so probably won’t work)
Other warehouses/versions
This item is available in 1 another warehouse/version!
1. Europe – DIY Spot Welder 9 Gears 18650 [EU] ($-3)
Price history and price alert
Here is price history (for all/most countries) for DIY Spot Welder 9 Gears 18650.
Price records source(s): 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)).