When we get orders, our shipping rates are lower than what we are paying. This is because the weights are not adding up when they change the quantity. We had an order for 35 screens (25 of one, 10 of another). The weight per screen is 7 oz but when they changed the quantity for one product to 25, the total weight stayed at 7 oz making the shipping rate incorrect and allowing them to pay just $7 for everything to ship via priority mail. Do you know why this would happen?
Do you have the weight set on the item? 7oz in lbs is 0.4375 ( Make sure the weight is correct). That being said, are you calculating real time rates for shipping? Would you like to review over a quick call?
If you attach a screenshot of your Shopify shipping setup we can likely identify what is causing this. It's usually a simple fix and also helps the community out if they come across the issue themselves - they can see how you resolved it :)
Shopify's support is pretty great, too! You can go on live chat or give them a call and they can review your setup with you.