I had never heard of Spax screws until I ran into a distributor at a big box hardware store stocking the shelves a few years ago.  I described my project and they picked out a screw for me.  It turned out to be perfect for the intended use and the self-drilling feature was a big benefit. I bought this variety box on Amazon and used several of a certain size, but it costs an arm and leg to buy either a small number of a certain size or a large quantity, so I ended up just buying another variety set.  That's my only complaint. If I could buy 100 of a certain size at a reasonable price I'd be happier.