Vouchers are a way of prepaying for a test at a discounted cost. The way it works is, a company buys test sessions in bulk from CompTIA and etc. with the intent of reselling them to you. CompTIA is happy because they sell the test sessions in bulk. The voucher company markets the vouchers and sells them to you. Because they bought them in bulk from CompTIA, Microsoft, etc., they get them at a discount. Some of the savings are passed on to you. Here's another post you may want to read.
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