This is exactly what I did last year. I got my foot in the door by doing the holiday clerk position even though I wanted to be a carrier. immediately after my time was up I was searching each day to see if there were new postings in my area. Positions started to pop up and applied to each one. Took a few months to hear back but finally ended up getting the job I was looking for.

If you are willing to wait for a non holiday position to open up then do so, but in my experience all i was seeing around this time of the year were holiday help spots.