I've seen this and considered this a number of times but quite often I don't think it makes a difference. Say the invoice is $1k... is that $30 savings really going to motivate them to pay it quickly?

Personally, I think you're more likely - no matter how bad it is - to have a better effect going negative....+1.5% for every 30 days late. It might be pretty good motivation to not let something run late.

My 0.02 -3%.