Not sure what you want to display in the invoice? If you want to show the selected value, just use the syntax [NAME_OF_FIELD_IN_UPPERCASE]
If you want to see the value of the fee associated with the field, use the syntax [NAME_OF_FIELD_IN_UPPERCASE_FEE_VALUE]
For example, if the name of the custom field is paid_option, then the tags are:
[PAID_OPTION]
[PAGE_OPTION_FEE_VALUE]
Please try that and let me know if you need more help
Regards,
Tuan
