Hi there @sillycube!
Don here from Shopify.
Thanks for sharing your query with us here in the Shopify Community.
I can shed some light on this for you based on my own experience.
It would definitely be common for prices in these countries to be displayed with two decimal points, representing cents or pence.
In fact, it would appear quite unusual to see a dollar or pound price with only one decimal place, as that is not the common convention for these currencies.
Is this your first time working on an app for Shopify or have you had some previous experience as a Partner or developer here?
I am working on my 2nd app, BYOB but it's much more difficult than the 1st one since my 2nd app involves order edit, discount & price calculation.
I tried to allocate the discount on 1 line item to multiple line items but the rounding issue makes it hard. I also opened a thread here.