For the record, they do have support for email warnings when your balance drops below various thresholds- it's right on the account dashboard.

I came to post the same. NFS does have some email alerts, but if you leave something on NFS and then forget to add funds you may lose it. That happened to me, and I decided never to use them again, because at the end of the day I’d much rather have infrastructure bill me and stay online than require pre-payment and die when the funds run out.

But if you’re sure you’ll never forget to top off your account balance, NFS is okay.

I’m pretty bad at keeping on top of that sort of thing, and their email alerts have kept me on top of it for close to 20 years now. I set email alerts at $5 and at $1 remaining and I almost always ignore the $5 one. I think I’ve gotten the dreaded “your account is out of funds” emails three or four times in that time, and I can have it back to running with a few clicks. I’m pretty darn lazy but I think their system is wonderful. No out-of-sight auto-pay that creeps up in cost, but still relatively easy to manage.