Nightyume I'm not sure what country you're in - but if you save your country to New Zealand and then exit Koboboooks.com .... and then use Chrome to start Kobobooks.com you'll find that you're in the new look Kobo and the new look checkout will work for you.
Once in the new look checkout you will get the option to use the various vouchers. Have a read of my posts above and see if they help.
Good luck
