Flat shipping rates charge a specific amount for shipping at checkout. You can create flat shipping rates for any order within a shipping zone, or only for orders whose weight or price is within a certain range. For example, in a shipping zone for Europe, you could set a flat rate of $10, and another flat rate of $5 for orders that are over $50.
When combining price-based shipping rates, Shopify combines rates that apply to the total price of the cart. When combining weight-based shipping rates, Shopify combines rates that apply to the weights of the individual products. For more information, refer to Combined rates at checkout.
For example, suppose that you have a shipping rate called Discounted shipping for $5 in two different profiles. This rate is available on orders over $50. If an order contains a combination of products from both profiles and has a total of $55, then the checkout shows the Discounted shipping rate at a cost of $10.
Instead, suppose that you have a shipping rate called Discounted shipping in two different profiles that gives cheaper shipping on orders over 5 kg. To receive this rate, customers would need to order 5 kg of products from the first shipping profile and 5 kg of products from the second shipping profile to qualify for the Discounted shipping rate.
From your Shopify admin, go to Settings > Shipping and delivery.
Next to the shipping profile that you want to add a shipping rate to, click Manage rates.
Next to the zone that you want to add a rate to, click Add rate.
Enter the name and amount for the rate.
Optional: Add weight-based or price-based conditions.
Click Done, and then click Save.
Refer this Link
Thanks for the explanation! I’m working with weight-based shipping, and trying to make sure my weights add up correctly at checkout. I’m trying to figure out whether to weigh my items by themselves (1oz for a print, for example) or the weight when packaged (4oz). Like let’s say someone bought 10 small prints at 1oz each. The calculation on Shopify would be 10oz total, but after packing up the 10 prints, my final weight including packaging would be above 13oz, moving from First Class rates to Priority Rates (yet the customer only paid for First Class). I hope that makes sense. If anyone has advice on how to weigh their items, I’d love to hear!