Why is my checkout displaying the wrong currency on my Wordpress site?

Good afternoon, I’m looking for help with a currency issue.

I’m using the Shopify Lite buy buttons on my Wordpress site, and have just reached testing stage.

My store currency is USD.

My payout currency is GBP.

The buy button product displays show correctly in USD.

However, there’s something wrong with the checkout, because

a) it rounds a USD 3.99 price to USD 4.00 in the subtotals

b) when I go to the payment, it shows in GBP 4.00 - which is clearly incorrect.

Would someone be able to point me to where I’ve gone wrong? (I have screenshots, but it won’t let me attach images…)

Many thanks

Sue