Shopify complaint: Additional shipping charges

I purchased a label to ship a painting through my shopify store. The painting was packed by my local UOS store which I have been using for the past year. UPS gave me the size of the box to input for my label and I purchased a label for $134.73. About a month later I see a charge on my cc for $922.50. After much time investigating, I am told that the box was 1 inch bigger. I talked to my UPS man and he said it was the size he gave me. I emailed support and they asked for images of the box with a tape measure to show the size. I had UPS take pictures of the box that they shipped the painting in with the tape measure as requested. Now they are still denying the credit. I could have had the painting couriered to the client for $400 and now this shipping charge totals $1057.23! They are still denying and I have asked to speak to a supervisor. Kelsey from shopify support says I need to take it up with UPS. I bought the label through shopify not UPS. Shopify should be the one correcting this on my behalf as a customer.