Shipping rates are calculated based on a variety of factors and not just the weight. The following factors are responsible for the calculation of shipping rates at checkout :
- Shipper address
- Destination address
- Weight of shipment
- Dimensions of the shipment
Shipping artwork might require extra planning and care, compared to other items. In most cases, the shipping cost of unframed and framed paintings varies considerably since unframed paintings can be rolled and packed into cylinder packaging boxes, making them lightweight and cheaper to ship. whereas, framed paintings can be larger in size, heavier. This might make shipping costs high and will take more time to prepare. I’m not sure what is the exact reason in your case. I assume it might be because of the packaging issue with Shopify. Since your product might require different box sizes, handling this directly with Shopify will not be ideal since Shopify allows you to choose only one default box for all your products.