Through shopify, it appears currency invalid when customers try to make a transaction.

Thanks for reaching out here in the Community, @info-trynoala-co. 

To get the assistance that you need, I suggest reaching out to the Shopify Support Team. They can check why the currency is invalid when customers try to make a transaction.

Here's how:

1. On your QBO account, go to the Apps menu on the left panel.
2. Under the My Apps column, choose Shopify.
3. Click the Support tab to get their phone number.

In the meantime, you can check out their Help article for more information.

Please let me know how it goes after contacting them. I'm only a post away should you have any follow-up questions. Have a wonderful day.