Ok, so I found that if I set my bid to 99 million sats/month, it will once per minute look to ensure I have enough sats balance to accommodate the maximum possible charge for that next minute (2,314.5 sats). If my SN wallet doesn't have that much, my listing will get stopped for reason NO SATS.

But if I did have 2,315 sats or more in my SN wallet, it would have passed that check, and only charged me a fraction of a sat (which is the actual amount my listing will cost -- regardless of my bid, as 1,000 above the next highest bid of 10,000 is 11,000, thus 0.25 sats per minute).

Now I better understand how it works.