Your best bet is to use their email contact as this involves a payment issue email@example.com
But some steps you can take yourself is to contact your card issuer in case it's an issue on their end. Assuming that JNC attempted to charge the card, your card issuer should be able to see what went wrong on their end (if the transaction was flagged as suspicious for example)
I'm in the UK and I seem to recall at least one instance where my bank blocked a payment to JNC and flagged it as suspicious (maybe because it was a foreign currency), so it would be good to get a heads up email.
I'm in similar circumstances. Where I live, USD is a foreign currency that's strictly managed and I could only purchase the subscription using a special dual currency card with certain hard limits. I suspect that time my bank called me isn't going to be the last and there may well be future instances where the payment gets automatically declined.
I went back through my own emails and I've never gotten an email about subscribing, only emails about purchasing extra credits. It would be nice to get some extra notifications, I'm pretty sure I've seen people indicate they'd like a notice about when credits are spent too since there's no real way to track any of that unless you're meticulously noting every time you spend a credit or get a monthly credit
I get an invoice email from my hosting provider (Digitalocean) every month, a similar email from j-novel would be enough for me. That being said, having more granular notifications would also be great!
@qu1nch1a Partly Yes. If it is expired you could either buy it to read it or wait for a catchup month. Every months some series are made free to read for members. Like Black Summoner plus 5 other LN and one Manga this month. So as a member you can read all Volumes of it.
J-Novel Club has used Stripe as its payment processor since it first started almost 5 years ago. There have been no changes on that front whatsoever. It's more likely that there's something wrong with your card, which you will have to call your bank or card issuer to investigate.