The ideal time is immediately after the payment fails. My bank would tell me about the failed payment immediately, and it would be useful to have a way to fix it without having to wait until whichever hour makes people 0.001% more likely to open e-mails.
I want e-mails in the same language as I use the service in, from the same e-mail server that already sends me transactional e-mails, and with the message content written by the people behind the service. An e-mail in the wrong language may be misunderstood or interpreted as a scam. An e-mail from a different server may end up in the spam folder.
Seeing your pricing should not require filling out a long survey and an e-mail address.
All in all, is it really a product? It seems like a feature that the payment providers would already offer, or maybe a slight improvement upon it.