You can have one flavour per tier from my flavours page without changing the price, unless it is fruit cake, which costs slightly more per tier than the sponge flavours.
If you really can’t decide, you can have two flavours in one tier (split half and half) without changing the price either. It is better if you choose flavours that go together if you are going to do this. (i.e. vanilla and chocolate rather than vanilla and carrot cake).