Commerce CGI - fix shipping cost mech

Need to adjust shipping options in CGI Commerce Cart but we are unfamiliar with the new script. In the old store (old CGI Commerce cart before upgrade) the shipping charges were calculated as follows: 15% for orders below $200 and 10% for orders above $200. In the old script, this was a fairly easy modification but this new script is more difficult because of the extra options offered. Please modifiy the shipping calculations to remove the customer specified shipping option and set it up like it was before.