Ann, I also made an expensive mistake. I bought some Merrell walking shoes and after a few times wearing them I realised I had bought a pair that were going to cause me strife on downhill sections... they were simply too small. I went out and bought a bigger pair. I thought at first that the next size up was simply too big. But having worn them a few times now, I know that with socks and a liner they are just right. It was an expensive mistake, but in the long run I am glad I went out and spent the money a second time.
I have backpacked in various places quite a lot, and have always 'got by' on a trainer kind of walking shoe, plus a sturdy sandle. But last year I climbed "Croagh Patrick" in Ireland, on a day when the weather turned foul. It has a very stony kind of surface, and once it was wet, I found I was slipping all over the place. The wind also became very strong. At one point I slipped over and was then rolled around on the ground by the wind. It was a very unnerving feeling I can tell you! I was fortunate not to injure myself more than with a few bruises.
What kind of shoe you wear on the Camino will be a personal decision, and the season you walk in will affect it. But after my Croagh Patrick experience, I have gone for a shoe with a very good non-slip Vibram sole. I have now worn these shoes enough to be confident that they provide very good ankle support over uneven rocky terrain, and I am no longer concerned that I need boots.