Creating label to print to Zebra thermal printer

[asalgan]

I got the labels to create a full-size PDF, but it shows up distorted when printing to the ZP500 Thermal Printer to print FedEx labels. How can I create the labels in such a way that I can print to a thermal printer? When creating the label I have:

fedex.label( ..., :label_specifications => LABEL_SPECS )

LABEL_SPECS = { :label_format_type => "COMMON2D", :image_type => "ZPLII", :label_stock_type => "STOCK_4X6" }

I've gone through the FedEx API docs and tried any and all combinations for the label specs with no luck.