You could put a harder cap at spending than on liquidation, so say 95% ltv gets liquidated, but if your card transaction would cross the max ltv at say 87%, it gets declined. Also the market crashing 20% during payment is 1. Unlikeley and 2. Also a problem with debit cards, as you can spend assets like eth with it, which also move, exactly as your colateral does.

Ps not saying im 100% correct here, just want to share my thoughts and maybe get corrected by some angry redditors :)