Ability to choose from multiple label sizes for Fedex

[ggiraudon]

**Is your feature request related to a problem? ** For certain carriers (like Fedex), there are multiple label types and sizes that can be used. In my case, my printer is set up for PDF_4x6_75 labels (with doc tabs) but the default settings in the dashboard map to PDF_4x6. This means that I am unable to purchase labels from the dashboard if I want them in the PDF_4x6_75 format.

Describe the solution you'd like It would be nice to have a way to select other label sizes in the carrier settings.

Describe alternatives you've considered At this stage, the only option I have is to modify the label mapping in the code directly.

Keep up the good work ! Guillaume