5V High Efficiency Waterproof Solar Panel (Banggood, Aliexpress: Short Circuit Current ≥ 560ma, Open Circuit Voltage ≥ 7.2v, Cell Conversion Efficiency ≥ 24%, Minimum Output Power ≥ 3.5w, Standard Working Voltage ≥ 6v, Standard Working Current ≥ 540ma (the best price with coupon)).
Basically copy coupon, view price history or create e-mail price alert. 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 code price check (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
1. ($18.05): 8BGwbg2204
2. ($18.25): 2203ActiesNL
3. ($18.44): BG00c906
put item in your cart and apply coupon code there after clicking “Proceed to checkout”. Click to copy code and open shop.
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/feV
Old coupons (from the oldest to the newest): 00c906, ADMITAD2212, Terence, Tabby20
(you can check these old coupons out, but these probably won’t work)
Price history and price alert
Check out price history chart (for all/most countries) for 5V High Efficiency Waterproof Solar Panel. The lowest price was: $17.66.
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)).