If you are using the built in MemberVault payment forms, you can easily add coupons and give discounts.  Here is the step by step on how to do it.

Edit the Product

Click on Products, and click edit next to the product you want to add the coupon for.  On the right hand side, fine "Payment Options", and make sure the "Sign Up Type" is set to "Payment".  You should have at least ONE payment option set.

Every payment type can have its own coupons.  So if you have one price in USD and another price in CAD, or want to offer a discount for a one time payment instead of the payment plan, you can do that.

In the lower right-hand corner find and click on  "(add coupon)"

Add Coupon(s)

Each time you click (add coupon) it will add a new row where you can add a coupon for that payment option.  Coupon names should be simple with no special characters or spaces.  You can choose between an amount off, or a percentage off.  If this is a payment plan or recurring, this discount will apply to ALL payments.

Currently there is no way to "expire" a coupon, but you can simply come in here, click the red X to delete the coupon and it will no longer work.

User's View

We've worked hard to make the the EASIEST coupon experience possible for your users.  Whenever you have ANY coupons active, the payment form will automatically show an optional "coupon" field.

The user only needs to type in the coupon code (in any case) and as soon as they're done typing, the discount is applied and the total is updated.  In the event your coupon makes the total $0, the user can complete the purchase without having to go through PayPal / Stripe (great for giving promo access)