Hello, thank you for taking the time to post your question!
Since he’s paying within 8 days that means that it’s going to fall within the 10 day window and he’ll get a 2% discount. That means we can take
Discount = Net Amount x Discount Rate = $430 x 0.02 = $8.60
Final Payment Due (after discount) = $430 - $8.60 = $421.40
I hope that helps you get moving in a better direction on this type of question! Feel free to reach out if you have any additional questions beyond that :)