Running boards take a beating from rocks, mud, and boots every single day. They make getting in and out of a lifted truck actually manageable, but buying them at retail price is a waste of money. You want to look for online retailers slashing prices during major holiday sales when the markup drops.
We are looking at about $400 for a decent set. You want to save your cash for the install or other gear rather than dropping it all at once.
That 0% window is a deadline, not free money. If you still have a balance left when those 18 months are up, the remaining amount will start getting hit with regular interest.