Hi @CB52 !
You are correct; 100 credits = 1 conversation = $1. You can purchase additional credit packs in quantities of 1000 credits ($10USD/pack) at any time.
You will only be autoupgraded into another $10 committment if you purchase more than the included volume of credits in your portal. If you do trigger an autoupgrade, it will just be consistent to the band of 1000 credits you hit.
If you have 5000 credits and you consumed 5500, you would be charged for 6000, in increments of $10.