I would like to suggest that the Voucher colour scheme are somewhat confusing.
The silver colour is easy to differentiate, however, the other 2 ‘goldy’ coloured vouchers are hard to tell apart.

An easy fix would be to reclassify the vouchers with clear distinguishing colour schemes, say:
GOLD / SILVER / BLUE (or some other colour your choosing)

I can only assume that the 2 goldy looking vouchers are gold and bronze.....almost same colour scheme.

Whenever I wish to use the vouchers, I have to employ a ‘trial and error’ option at time of purchase to differentiate between theses 2 vouchers in order to complete the transaction.

Many a time I have made an error and used vouchers not intended for use.
It’s no big deal, but I do believe that due consideration is worthy in this regard.