A. B They don't call it ONE SIZE FITS ALL, they call it ONE SIZE. There are still measurements on the website for you to guide yourself. If it doesn't fit you then it's your fault for not checking the measurements.

I had the same problem. The email says % off your purchase not the original price of item. Then what's the point of the coupon? Will probably not purchase from site.

Dear customer,
Nice day.
Thank you for choosing SheIn.
Our web has mentioned that the coupon discount is based on the original price of each item. If the item is on sale and has a larger discount than the coupon discount, then it will use the sale discount instead of the coupon discount.
Hope you can kindly understand.
If you need further assistance, please contact us via dispute@shein.com.
Thank you for your support.
Best wishes.

Same thing happened to me. I had a code for extra 40% off, but it didn't take it off. The all all one size fits all, doesn't. Would not buy from this site again.