That IS frustrating 🙁 I’ve had that happen to me, too. While I could have used either size, it’s still not what I ordered and paid for.
If you’re comfortable with the size and the capacity, then you can give it a try. It’s totally up to you if you want to keep it or return it.
Size B = before birth? The smaller size? The MoonCup or Keeper? The Keeper was the cup that I received the wrong size 🙁
If one of these is the cup you ordered, the difference between the sizes is so small that it might not make a difference with this cup.
In the end for me personally, the capacity of the cup wasn’t what I needed 🙁