Hello @Hibiscus and thanks for your post. I didn't even realize it said "voucher". I do not see a place to customize this text, so I think this not editable (only have the option to customize gift card designs). I guess the best thing you could do is use the wording you'd like directly in a Facebook post, and then copy/paste the link to purchase the gift card. Perhaps your customers will not see the "voucher" text, or you could use the same language in your post...ex. "e-gift card/merchandise voucher" or something like that.
Here is some more info about setting up e-gift card if you're interested in reading more.
