Hi, Deborah.
Mrs. Flynn and I have run into similar problems when buying Sim Points. Usually, it's when we've already purchased one bundle of points and are trying to buy a second bundle. Wait 24 hours and try buying the Sim Point bundle again, or try it with a different card, if you can. If all else fails, see if you can find Sim Points cards at places like WalMart.
We've also run into a similar issue buying items from Sony's PlayStation Network. The issue there was the card we had didn't support a verification protocol that Sony uses for transactions, so they would always fail. We ended up purchasing PlayStation cards (similar to the Sim Points cards) to buy things there.
Hope this helps.
Flynn