Tip: Before you get started, we recommend taking a look at the instructions provided by Shopify first.
Charging an exact flat rate on Shopify is quite challenging. Below we suggest an alternative solution which, although not perfect, works in most cases.
Say you sell posters in 2 sizes - 30x40cm and 70x100cm. In order to match our flat shipping rates, you would need to assign an arbitrary weight to both these variants on Shopify.
For example, you set that 30x40cm variant weighs 50g and one 70x100cm weighs 500g (although this is not the actual weight). Then in Shopify shipping settings, you would set that for EU orders with a weight between 50 and 499g shipping should cost EUR 5.10 which is our shipping rate for 30x40cm posters within the EU.
Then you would create a new shipping tier stating that for all orders with a weight between 0.5-1 kg shipping price should be EUR 5.55 which is our shipping rate for 70x100cm posters.
If you had an order for ten 30x40cm posters then it would automatically calculate the order total weight as 500g and charge the next tier, i.e. EUR 5.55. This is not optimal, but unfortunately, it is the way Shopify works at the moment.
Learn more about setting up shipping rates on Shopify FAQ.