Hi there,
I am an oscommerce programmer of over 20 years and I can take of the coupon issue that you are having. Since you will not allow ftp access there will be more information that is needed besides just the files to efficiently and properly fix your issue.
What version of oscommerce?
Which Coupon Module Are You using?
Which version of php are you using?
What is the site url?
Can you give me a sample coupon code so I can see the issue first hand?
Thank You for your time and consideration,
Shawn Mulligan