It doesn't work Cath, have tried it myself with the help of Shopify experts.
This is a serious flaw with Shopify, and they have no interest in fixing this fundamental issue.
All we are asking for is to show the shipping costs before the customer goes to that infamous PayPal Express checkout. How hard is that to implement?
It works perfectly with Volusion and other eCommerce platforms.
It's quite extraordinary that Shopify have ignored this issue for so long. Incredible to say the least. The mind boggles.