The site is in Australia so if you’re not based there you may need to negotiate shipping costs by email/phone first.
As a guide I usually wear 34”/86cm trousers and have purchased the Size 20 shorts, the exception being the red ones which I found in a shop and were Size 22 (I guess these sizes an extension of age-based sizes e.g. size 14 for 14 years of age).
I’d probably add or subtract one inch (2.5cm) for each single difference in size, e.g. if size 20 = 34”/86cm then size 18 is the equivalent of 32”/81cm and size 22 = 36”/91cm. The waist of course will stretch somewhat. Hope that helps :)