1. It's a bad idea.
  2. No amount of effort and consideration will lead to her expressing appreciation.
  3. Show the sales associate a flattering pic of the woman for a size estimate. Buy something 2-3 sizes smaller than recommended.
  4. Some aspect of my purchase will offend her, likely in a deeply grievous fashion.
    It might be expressing shock that it's too big being that I "bought something three sizes smaller than the girl suggested."
  5. Forget alterations: this attempt to do something nice will end up requiring reparations.
  6. Flowers are always a better idea
    Pro tip: Set up the IFTTT recipe "IF I'm at (name of mall), THEN send me a push notification to not but clothes and B line to the florist."