No resolution or help for USPS shipping error +overcharge

[mani-muse]

When purchasing a label in Shopify my Apple Magic Mouse sometimes scrolls up and changes the weight when purchasing a label. Long story short, this happened when purchasing a label for 3 kits of Press-On Nails that weigh 1.6 OZ each. Instead of the weight being 5.9 OZ, it scrolled to 72 LBS. I contacted Shopify to kindly adjust the error and refund the difference. When doing so, I provided Shopify with the package dimension, images of the items on a scale, and also duplicated the order to show the correct weight. Shopify responded and said that they contacted USPS and that "The dimensional weight is calculated by multiplying the package's length, width, and height and dividing by a carrier-specific volumetric divisor." The is no way 72 LBS is acceptable for a small padded mailer that weighs 5.9 oz in total. 

Based on all the information, having access to all our orders that usually cost no more than $5.00 to ship, and evidence, Shopify is unable to help or will to issue a refund on the difference. Any advise on how to get this lavel corrected?