I have one supplier who offers me 3% off my invoice on top of whatever I have negotiated as a price or however much I have had thrown in to sweeten the deal.
It is payment with 7 or 10 days (can't remember) but on a £5k order that is £150 off and I spent that the other week at the local pizza place. Seems far better to me to be in my pocket than theirs.
Some suppliers give discount for pro forma, some go 10/14 days, some end of current month but most are now end of current month plus 30 days.
I pay whenever it suits, always within terms but I am certainly incentivised to pay early for even a few percentage off. It is worth thousands to me each year.