I am monitoring price for ZK-MT21 Bluetooth 5.0 Subwoofer Amplifier Board and it is for $12.99 now (in Banggood). Just check coupons out (with price alert and history). Let’s see what best price is ATM.
Latest coupon code price check (UTC time):
ZK-MT21 bluetooth 5.0 Subwoofer Amplifier Board 50WX2+100W 2.1 Channel Power Audio Stereo Amplifier Tone Board Bass AMP AUX
Banggood ($12.99)
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. You probably want to know if this is a good price. Well, this price is higher than average price ($11.84)
Link: Banggood product page
Price: $12.99
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): BG589b2e
(you can check these old coupons out, but these probably won’t work)
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Price history and price alert
Here is price history (for all/most countries) for ZK-MT21 Bluetooth 5.0 Subwoofer Amplifier Board.
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)).