Orders shipped to the UK are sent using Royal Mail Tracked 24, £4/€4.5/$5*.
Worldwide orders use Royal Mail International Tracked, with signature where possible (not all countries support signatures), £14/€15.5/$18*
Shipping on orders over £100/€111/$130* are free.
Prints, as opposed to plots, are sent separately by The Print Space, which charge a flat rate of £7/€7.75/$9 to deliver outside of the UK.
If you buy a combination of plots and prints, Shopify will somehow work out a shipping figure based on goodness-knows-what mystical calculations, your guess is as good as mine.
*Approximate conversion rate.