How to apply coupon before checkout using Cashier?

[TheDoctorD]

So I have the user at a checkout page, that shows the price of the subscription. I want to have them enter in a public facing promotion code such as SAVE50, have them apply it, and show them what they are going to save before checking out. Is there a way to do that with laravel cashier?

If there isn't a method for this with cashier.. do people usually calculate the discounts themselves? I was running into small problems such as getting my discount calculation to perfectly match up with what ends up in the invoice.

I didn't see anything about applying a coupon to a price before checking out.
I only saw the method to apply the coupon to a subscription when it gets created:

$stripeSubscription = $user->newSubscription($product_title, $stripe_price_id)->withCoupon($subscription_product_coupon->coupon_stripe_id)->create($payment_method_token);

Thanks for any insight.