The 'chosen for price not anything else'* provider of my home SSL cert sent me an invoice that said 'Paid', which was very confusing when my SSL cert never got renewed. Turns out my credit card had expired and I'd forgotten to update it (well, ignored the email as I thought 'I'll do that when they bug me at renewal date').
Never once got a communication saying payment was declined, but hey ho, turns out Lets Encrypt wasn't much faf at all... their loss.
* shame really, as they were easy as 1....2... 3...