In between Ruffwear Flagline harness size

I’m currently visiting the States and looking to get the Ruffwear Flagline harness for my boy back home.

Thing is, his chest measurement is 55cm, and the XS looked too small, while the S looked too big!

It’s too expensive for me to wing it, since I wouldn’t be able to bring it back to exchange. So would like to ask any owners who have the Flagline harness, and may advise with regards to which size I should get?