UPS delivery package service select

Hi guys:
I’m using gem shipping for my project,i wanna select UPS delivery
package service, but it doesn’t work for me whatever value i send to UPS
api.
the following is some code i used.
UPS_SERVICE = {
‘UPS Next Day Air’ => ‘01’,
‘UPS Second Day Air’ => ‘02’,
‘UPS Ground’ => ‘03’,
‘UPS Next Day Air® Early A.M. SM’ => ‘14’
}

shipping_params = {
:zip => customer.shipping_zip,
:state => customer.shipping_state_province,
:sender_zip => SystemSetting.named(‘Shipping Zip’),
:sender_state => SystemSetting.named(‘Shipping State’),
:weight => items_weight,
:code => session[:code] || “03”, # default UPS
service: UPS Ground
:prefs => “#{RAILS_ROOT}/config/shipping.yml”
}

any guys have any good idea?
thanks.