Hi David,
Taxamo is a pay as you go service - £2 per transaction, no subscription but I do not know of any others. I guess if an integration app is developed then that will be another monthly fee… A developer has contacted me regarding this post and I’ve asked them to have a closer look at how it can be integrated with shopify, they are suggesting it may not need access to the checkout so fingers crossed. I will update this thread once I have heard back from them,
cheers Amanda