It’s clear for all to see that you are annoyed, even angry.
But don’t be. A voucher is actually a good option, if somewhat inflexible. So don’t accept until you are ready to purchase something, as you will have to use the whole amount at once.
Otherwise you will continue to upset yourself. It’s not worth it. You are in Thailand to relax; so relax. It’s only money. Your nerves and health are worth much more. Go to do something more relaxing and breath it out.