How to add a discount option on the payment form?

and able to choose discount percentage to apply

Hi Diana,

It's easy to add coupons for your products using the Coupons feature in Form Builder. Let me show you how to set that up:

1. Open your form in Form Builder and click on the Product List field.
2. Then, click on the Shopping Cart icon on the right side of the field.
3. In the menu on the left, click on the Coupons tab, and then click on Create Coupon.
4. Enter a Coupon Code and choose a Coupon Limit and set the amount or date to arrange the coupon's expiration.

5. In the next section, select a Discount Type and enter the Rate or Amount you want to use.
6. In the Apply Discount To section, select the Product option to apply your coupon to your selected products.
7. Then, in the Discounted Products section, select the products you want a discount for.
8. Now, just click on Save and that's it. You're done.

You can also apply a discount directly to the Subtotal or only Shipping. Let me show you how to do that: 

1. In the Apply Discount To section, select Shipping or Subtotal.
2. In the next section, enter a Minimum Subtotal Amount. 

Now, your customers can use your coupons to get a discount when they buy your products. Check out the screencast to see how it'll work:

Give it a try and let us know how it goes.