$9.99 – Material: Metal, CC: Output current adjustment, Output Ripple: ≤50mV, -IN: Input negative, VO+: Output positive, Working Tempreture: -10°C-75°C, CV: Output voltage adjustment, VO-: Output negative, Output Current: 20A(max.) / 15A (suggested), Color: Silver, EN: Enable / low voltage level shut off output; suspension / high voltage level is effective, Input Voltage: DC 6-40V (10V to 40V is suggested), When input and output is common grounded / no-load CV output constant voltage / CC output constant current, Output Voltage: DC 1.2-36V, Efficiency: 97% (24V to 12V / 20A), Size: Approx. 60x 52x 27mm / 2.36x 2.05x 1.06”, +IN: Input positive, Wiring Method: Terminal, Short Circuit Protection: Self-recovery (cannot short circuit for a long time).
You can get DC 6-40V To 1.2-36V 300W 20A Adjustable Step Down Module Board for $9.99 using coupon code (in Banggood). Go down and check coupons out. Remember to check price history if it is available. Let’s see what best price is ATM.
Latest coupon and price update (UTC):
Banggood ($9.99)
Banggood warehouse is located in China. I’m afraid that there’s no working coupon in Banggood. Is it a reasonable price for this item? Well, this price is higher than average price ($8.12)
Link: Banggood product page
Price: $9.99
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/i1u
Old coupons (from the oldest to the newest): 2022ACTIESNL, 8Couponsde, MVC8SPRING, 8BGwbg2204, 2203ActiesNL, 8wEbgaInS5, pROMo8cuPoN, BGJUNEGSG9, TTSUMMER8, Cuponation9, BG00c906, BG589b2e
(I don’t think these’re gonna work, but you can try ’em out anyway)
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.
Price history and price alert
Check out price history chart (for all/most countries) for DC 6-40V To 1.2-36V 300W 20A Adjustable Step Down Module Board. The lowest price was: $5.99.
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)).