In Sufficient Permission Error

I have a user event script which has available without log in and execute as Admin check boxes are checked. On customer center Make a Payment Transaction form i have a custom button, on button click my user event script runs.The script add line item to invoice,but the script returns the following error "In Sufficient Permission: You require a higher level permission on Transaction->Invoice.Please contact your Administrator". I have checked and found that Customer Center Role has view permission on Invoices.I also tried to customize the Customer Center Role and found that the max level of Permission to Invoice is