today I learned how to use your own fork in composer when the maintainers don't accept pull request.

First set the version you in the required section to "dev-master" , composer some how assumes you want to require the master branch.
Second add your fork as a repository to your craft/composer.json


{
  "require": {
        ...
       "nmaier95/shopify-product-fetcher": "dev-master",
       ...
   },
   "repositories": [
      {
          "type": "vcs",
          "url": "https://github.com/toyflish/craft-shopify-product-fetcher"
      }
  ]

Last update your bundle


composer update

use patched plugins in craft with composer