Victoria here from the Guru team! The best way to set this type of shipping up would be on a per product basis. If you set up your shipping rates per product you would be able to do $0.5 for a $10 dollar product, $10 for $100 etc. If you wanted 10% for shipping. The best app for this is called Better Shipping.
Otherwise, you will be able to set up your shipping rates based on order price, rather than a percentage of the price. You can use a set up such as; Orders between $5-$10 = your shipping rate.
Hope that helps clear everything up :) Let us know if you have any further questions.
Victoria - Shopify Guru
I agree. I think this is an enormous shortcoming of Shopify. How hard can it be to add an option so that the shipping is a percentage of the total price. My business sells hand-made ceramic items that cannot be categoriezed in weight or shipping boxes because we have thousands of different items of varying shapes, weights and sizes, that need to be packed differently on a per order basis. There is no way we can define shipping rates based on weight or box size because we cannot know these parameters until the item(s) are actually boxed and ready for shipping. A percentage based shipping cost is absolutely necessary for our business. Also, not just one percentage, but various percentages are necessary for each shipping option that the customer can choose from; for instance: Ground Shipping, Priority Shipping, 2nd DAY AIR Shipping and NEXT DAY AIR Shipping - each with it's own customizable percentage of the total sale.
It's amazing Shopify has not done this yet; and there is no plug-in that offers this. I like Shopify a lot but this is such a problem for my business that we are considering other options other than Shopify to setup our online retail store. I hope they implement this VERY soon.
Please do make this a priority for your developers. Thank you.