If it cost 7 dollars to ship an item, I could have sworn my store use to increase the shipping cost by 7 more dollars once a weight limit was exceeded, and then 7 more dollars once the weight exceeded the second package. It's no longer do that. I have carrier-calculated shipping on, but there is no way that my test with 15 shirts would cost 7 dollars to ship. Any ideas?
Shopify shipping calculation not that flexible to set shipping rates, but you could setup based on clothing weight manually when you import product info, for example, t shirts 0.25kg/pcs, dress 0.35kg, sweatshirt 0.5kg, sweater 0.8kg(might calculate volume if exceed 2kg), then you fill shipping rates in different countries or apply free shipping if you sell middle or high price clothing.
Have you created any weight-based or price-based shipping conditions?
If you have created a weight-based shipping condition,
1. For the max weight of the package, $7 shipping charges are applicable.
If you are looking to charge $7 for each shirt added to the cart, you can accurately weigh the weight of the shirt and set the condition as above.
If you want to charge only $7 as the total shipping charge for all the 15 shirts then add the shipping condition based on weight, i.e
total weight of 15 shirts(this will be the maximum weight to set the condition)