Welcome to the forum.
What is the Frame Size that you are choosing in Export?
Your AVCHD material should be Square Pixels, so PAR = 1.0 should be the best, but you will need to choose a setting that matches the Aspect Ratio of your footage.
In most cases, if you use one of the default export settings, rather than trying to tweak your own aspect ratio, you will get a 4:3 ot 16:9 with no letterboxing.
I'm pretty sure the the pixels are 1.33 because I've been able export in other programs with the following results: 1.0 pixels yields a 4:3 aspect ratio that looks squished and 1.33 pixels yields a 16:9 ratio that looks correct.
With Premiere, I've tried the following settings:
- NTSC DVD Standard (720x480)
- NTSC DVD Widescreen (720x480) (1.212 pixel ratio)
- MPEG2 1440x1080i 30 (1.33 pixel ratio)
- DV NTSC Standard (720x480)
- DV NTSC Widescreen (720x480) (1.212 pixel ratio)
- HD 1080i (1440x1080) (1.33 pixel)
- HD 720p (1280X720) (1.33 pixel)
- LAN (640x720)
Some others I can't remember at the moment. The widescreen settings letterbox both top and bottom, so again, I'm pretty sure that's the right dimension.