My size online is either 30E or 30DDD; they're supposed to be the same size. One is the American size and the other is the European size. I can't fit a 30DD; the cups are always too small. When I was measured in a bridal/specialty bra store, my size was 30E. My actual bustline was 28.5 when measured, but the cup size bumped it up to 30.
I've also returned a bra and a bikini top to either Bare Necessities or Figleaves and I had no problem. Their return policies are very good.