I'm pretty sure all the vouchers are only valid for the first 14-30 days an account is open and only one voucher can be used per account. (You are only 'allowed' to use one voucher/code per person as well.)
If you really think Google won't cross-reference the details used to apply for the vouchers with those used to redeem them then go on buy them... otherwise, just spend fill in the form and wait a few days for the code.