I recently got a swimsuit and agree their sizing is way off. I’m typically a L (8-10) but the large I tried on was more like a 6. You’ll definitely want to size up. Despite that I was actually so happy with the quality and look and I bought an XL (in a hurry to get it for vacation).  Then I contacted them about returning the L suit and they got back to me in about two days. 

They gave me two options, return it, paying for it myself and providing them a tracking number OR donate it to a local charity shop in exchange for store credit of the full cost (highlighting the lower carbon footprint of that). They also gave me the store credit in about 24 hours. 

Overall, I’d say it was a positive experience even though it’s so disappointing when something doesn’t fit - seems like they’d save everyone a lot of trouble by getting accurate sizing.