I have tried the S, SM and CHP Ewas and tbh, they all work for the monoboob. I think in most of them the 30G is a standard size actually. I the S wisniowska was my first and is fabulous, I have an S Fresh which runs fractionally smaller for some reason. The SM is unpadded and quite open at the top so no quad boob in it, and the CHP is more of a half cup so slightly wider and with wider strap placement.
Make sure you keep every single bit of paper in the box for return purposes - they have some odd rules about receipts and returns and I nearly came a cropper. However - pay by paypal and that way you have backup ;) Joys of international shopping
Sm Trefl - I have this in a 30HH and it still looks like the smaller version (i.e. doesn't have the extra panel
s wisni
These both show as in stock so are returnable. I think it's if you have to email them to request a size that isn't on the dropdown that its a custom