Preview a coupon for single-use in the next invoice for a plan that has a trial

When it is necessary to preview a coupon discount intended for single-use on the `next invoice` for a subscription whose plan has a trial, and then expire the coupon, the coupon should be configured as follows:

Discount: xx% (or fixed discount)
Duration: Forever
Maximum Redemptions: Unlimited
Redemptions per account: 1
Eligible Charges: All plans (or specific plans)
Discount level: Subscriptions

A Maximum Redemptions of unlimited is required in order for the coupon instance to be reflected in the next invoice pricing preview. A single-use coupon does not have awareness of future invoices and would apply only to the current invoice of the pricing preview (i.e. the `now` pricing).

The Redemptions per account setting of 1 has the effect of overriding the Maximum Redemptions setting of `unlimited`. This ensures the coupon will expire once it is redeemed on the next invoice after the trial and behave like a single-use coupon on the `next` invoice.