I don't know the exact database structure, but that seems like over complicating things.

Have you tried doing the below for the products:

RewriteRule ^product/([\d\/]+)/([\d\/]+)/(.*) /store.php?crn=$1&rn=$2&action=show_detail [L,I]

so
http ://www.westerndrugmedical.com/store.php?crn=210&rn=389&action=show_detail

would become:

http ://www.westerndrugmedical.com/product/210/389/Guardian_Easy_Care_Folding_Walker