I do two different types of voucher.
Where a gift voucher has been purchased as a gift for someone, the terms and conditions are:
1. Valid for 12 months from date of purchase. Dates of purchase and expiry are visible on the voucher.
2. Voucher can be exchanged for single spray tan, put towards a course of tans or used against the purchase of retail items.
The money taken from the sale of the voucher is put aside and not used until the voucher is redeemed.
Occasionally I distribute £5 off vouchers (I did this last December, valid for spray tans carried out in the new year). These are clearly marked with "valid until xx/xx/xx against the cost of a spray tan, cannot be used in conjunction with any other offer, may not be used towards the cost of retail goods".
Throughout the last year I've donated Spray Tan Vouchers to some fundraising events to be used as raffle prizes. These have a 3-month expiry date on them, are valid only for spray tans and not retail products, and may not be used in conjunction with any other offer.
All vouchers I distribute, whether purchased, used as incentives or as fundraising donations are numbered, logged and ticked off as and when they're redeemed.
Hope that helps.
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