What do I call the person that redeems a certificate? Could it be a "redeemer"?
EDIT 1 For example, a gift certificate for a spa, like a voucher...
"redeemer" would be good, except that among Christians it could be taken to mean "Redeemer" (note capitalization), a title reserved for Jesus Christ. Few other likely one-word replacements exist. "Claimant" is close, but can be used for claimers of other things besides a discount. "Tenderer" is accurate but rarely heard (the person offers the certificate as "tender" for part or all of the cost of service at the spa, just as they might "tender" cash or a credit card.) "certificateholder" applies only to the moment when they turn it in, so that does not include the moment of redemption. You might just have to use more words, e.g., "redeemer of the certificate" (and, for the cashier, the even bulkier locution "person accepting a certificate for redemption".)
As defined by Google:
a person making a claim, especially in a lawsuit or for a state benefit.
"one in four eligible claimants failed to register for a rebate"
It identifies the person claiming the benefit to which the certificate entitles them.