[checkout] Precent of item instead of flat rate for postage.

I was curious if there is a way to do a precent of item price instead of flat rate for postage.
Example: Photo =$40. Postage = 15% Total - 40*1.15=$46
Is there a way I can do this? Or code it in?

Gallery version = 2.1 core 1.1.0
PHP version = 5.0.4 cgi
Webserver = Apache
Database = mysql 4.1.18-standard-log, lock.system=flock
Toolkits = Exif, Getid3, ImageMagick, NetPBM, SquareThumb, Thumbnail, Gd
Acceleration = none, none
Operating system = Linux serv01.siteground118.com 2.6.14.2-grsec-grsec-test #1 SMP Fri Nov 18 02:27:52 CST 2005 i686
Default theme = matrix
Locale = en_US
Browser = Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.12) Gecko/20051010 Firefox/1.0.7 (Ubuntu package 1.0.7)

thanks!!