Here’s an example. Say you sell two sized posters - 18x24 and 24x36 and offer to buy them framed as well. In order to match Printful shipping rates you would need to assign arbitrary weight to 18x24 and 24x36 variants on Shopify.
For example, you set that 18x24 variant weighs 0.1 lbs and one 24x36 weighs 1 lbs (which in reality it doesn’t). Then in Shopify shipping settings you would set that for US orders with a weight between 0.1 lbs up to 0.9 lbs shipping should cost $7.95 which is our shipping rate for 18x24 posters within the US. Then you would create a new shipping tier stating that for all orders with weight between 1 lbs up to 10lbs shipping price should be $9.95 which is our shipping rate for 24x36 posters. If you had an order for ten 18x24 posters then it would automatically calculate the order total weight as 1 lbs and charge the next tier - $9.95. This is tedious, but unfortunately the way Shopify works at the moment.
Learn more about setting up shipping rates on Shopify FAQ.