Pro sites allows use of coupons when user does not have any time left and pro sites is expired.
However, when a user is in trial or manual extention of trial, and a user purchases a Pro Sites upgrade, applies a coupon code through pro sites, and submits payment to Stripe, it creates an invoice instead of an instant charge. On the invoice, it makes the regular priced item with disregard for the coupon.
It instead applies the coupon to a $0.00 instant purchase.
It gives it a gibberish coupon code as well. Notice, the coupon applied on Pro Sites page was "110" off, for $110 of a $210 order; in stripe it come serialized, as gIIUO6wV for 52% off
Stripe coupons dont match pro sites (not sure if thats intentional)
Anyways - it applies coupon to instant purchace instead of pending invoice.
Here are some screenshots of the invoice, and the POST logs from stripe