I have never had to use them directly but the use-now-pay-later model feels scary to me for the same reason. Maybe they allow setting the upper cap to the month

I have never had to use them directly but the use-now-pay-later model feels scary to me for the same reason. Maybe they allow setting the upper cap to the monthly bill (crossing which they don't serve you until you intervene) but I have never heard of it. On the other hand there are many stories extremely ballooned bills for some unforeseen reasons.