Hi there, we are running Printout Designer app that allows you to create DYMO compatible labels and then download them as PDF so you can print them using your PDF reader instead through the browser. Browsers can add extra margins, headers and footers and mess up your label size.
You can try out the app for free for 14 days: https://apps.shopify.com/printout-designer
All the best,
Tanel from Printout Designer