I have just checked coupons. The lowest price I could get is $27.89 for 5V 15W USB Solar Panel (in Banggood). Price history, coupons and price alert are available on this page. What is the best price?
Latest coupon and price update (UTC):
Process Laminated Sanding Board, Packaging Box Specifications 200*140*40mm, Current 0-2.4A, Conversion Rate 19%, Folding Size 170 * 120 * 30mm, Weight 400g（With Packaging Box）, Product Name 15W Solar Foldable Panel, Waterproof Grade IPX6, Output Voltage 5V, Unfolded Size 790 * 165mm, Output Interface Dual USB, Chip A-grade Polycrystalline Silicon
Banggood ($27.89)
Banggood warehouse is located in China. I’m afraid that there’s no working coupon in Banggood. Price? Well, this price is higher than average price ($26.07)
Link: Banggood product page
Price: $27.89
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): BG589b2e
(system checks these coupons on every price check (like once per 24 hours), but you can check them out anyway)
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
Do you need price history chart (for all/most countries) for 5V 15W USB Solar Panel? There we go.
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)).