PDF field: Change State/Province name to abbreviation

I have a field in a PDF that is calculating the value of two fields: Billing City and Billing State/Province with a comma between:

event.value=this.getField("Billing City").value + ", " + this.getField("Billing State/Province").value;

The information is imported into a PDF with a txt file where the State/Province is written as the full name, but I would like the above field to use the abbreviation.

I have the state/province field as a dropdown with abbreviations as the export value, however it’s still calculating using the state/province full name. I tried this code but then the State/Province field gets stuck on the first option:

event.value = this.getField("Billing State/Province").valueAsString;

this.getField("Billing State/Province").setItems([["Alberta", "AB"], ["British Columbia", "BC"], ["Manitoba", "MB"], ["New Brunswick", "NB"], ["Newfoundland and Labrador", "NL"], ["Nova Scotia", "NS"], ["Northwest Territories", "NT"], ["Nunavut", "NU"], ["Ontario", "ON"], ["Prince Edward Island", "PE"], ["Quebec", "QC"], ["Saskatchewan", "SK"], ["Yukon Territories", "YT"]]);

Is there a different way to change the state/province name to abbreviations?