1) This older version of ASPDNSF has got so many customizations. If we upgrade the site to latest version, do we lose all those customizations? How best i can upgrade but still keep all customization? is there a safe way to do it?
Not really. If you've made customizations to xmlpackages, any of the web code files, or any of the projects/classes, those will be lost during the upgrade. The only way for them to carry over is to reimplement them in the new files (I'd be willing to bet that 98% of all files in AspDotNetStorefront have changed in some way or another since version 6, especially with all of the PABP (now rolled into PA-DSS) changes). Should you go this route, hopefully you have your changes well documented or can compare the files against a clean solution so you can pinpoint your modifications.
2) I found this blog online
http://www.hosca.com/blog/post/2009/...-CertiTAX.aspx
If i get the web service from AVATAX, is it possible to do it this way as mentioned in this blog with out doing any code change or upgrading ASPDNSF to latest version (I dont care about performance right now) ?
Provided you were able to connect to their service to retrieve rates and store them in the table provided there I don't see why not. The integration we're rolling out with Avalara is a real-time rate solution, where the shoppingcart items are passed to their service along with the address(es) of the purchasing customer and the origin address you have configured, in order to retrieve rates in real-time during the checkout process.
<a href="http://www.aspdotnetstorefront.com">Shopping Cart Software</a>