I gave up giving vouchers due to the short time scale, i.e. had to be spent within 6 months on many. Also if the Company goes out of business the money is lost. Better to send a cheque in a pretty card.
I don't think that a voucher would be seen by machinery, however .......... with the extra staff on duty

. Many years ago I sent two cheques via Royal Mail to my nephew in Cardiff at different times, neither arrived !!!!