Salesforce Vouchers

If I buy a Salesforce Voucher, can it be used it on any Certification? How do I use my Salesforce Voucher? Do you offer retake Salesforce Exam vouchers? What happens if my Salesforce Exam voucher expires? What should I do if I can’t find my voucher? Where can I locate my voucher? What is the validity period of a Salesforce voucher? For which certifications is the voucher valid? How much does the Salesforce voucher cover? Can Salesforce Exam Vouchers be extended?