You can purchase AWS Certification exam vouchers by level for any corresponding exam; you do not have to specify the exam.
AWS Certification Exams
Choose from Foundational-level, Associate-level, and Professional-level or Specialty. You cannot buy a practice exam or beta exam vouchers through Xvoucher.
AWS Certification exam vouchers are purchased at an exam level—not for specific exams—which provides greater flexibility.
For example, if you want 10 of your employees to take exams for Associate certifications, simply buy 10 Associate-level vouchers. They may then choose which certification exam they would like to take within the Associate certification level (either the Solutions Architect, Developer, or SysOps Administrator exam). You don't have to know of time ahead who will take which Associate exam.
AWS Certification Roadmap