Doorcard Fastener kit and pilot holes for various cars starting at £15 for both doorcards
These are not the cheapest but they are much better than screws or rivits. They expand as you tighten them up which means you can also unscrew them and take off the door cards with ease and use them again. They are supplied with Stainless Steel Fasteners. I also then drill the pilot holes in your doorcards if you add this option.