My design integration was done rather easy.
What I have done:
Shop-Script is bases on the smarty template engine, I have not used any default template, but created one for my shop.
I copied my main site's template index file over the forum's main template file (include/template/main.tpl). I then got rid of all the unwanted elements out of the file (like shop-script or smarty tags) and replaced them with punbb's main template codes, keeping the rest of the file intact.
All I had to put in the new file is:
<pun_head>
<pun_status>
<pun_announcement>
<pun_navlinks>
<pun_main>
<pun_footer><pun_main> and <pun_footer> have to be within
<div id="punwrap">
<div id="pun<pun_page>" class="pun">
and
</div></div>
I have then put all of the elements mentioned as code above where I wanted them in the new template file.
The last thing I had to do was to modify some image and css paths, to have it work with the same files as my shop template.
Last was to fine-tune the forum css.
Hope I was somewhat clear ...