The Cambridge English Dictionary gives the following definition of the word voucher
a piece of paper that can be used to pay for particular goods or services or that allows you to pay less than the usual price for them:
In other words, the word voucher refers to something (paper, online message or card) or that stands for payment for all or part of particular goods or services, usually from a particular store. So when foreign travel became difficult or impossible in the Spring of 2020, many airlines, hotels and other travel bodies provided what they called vouchers to those who had to cancel. fifty years ago, this would have been done by the issue of a piece of paper or cardboard with the name of the company and the extent and limitations of its coverage, both in money and in what the money could by and the date at which it would expire.
It is derived from the verb to vouch, meaning, again according to Cambridge (UK, by the way),
to be able from your knowledge or experience to say that something is true
So I take it that the issuers of a 'voucher' which is a kind of promise that, subject to whatever terms and conditions are set out on the voucher, they will accept it as payment for the goods/services specified.
As I say, 50 years ago this would usually take the form of a physical printed piece of paper or cardboard. Now paper has been replaced by electronic records and messages exchanged between business and client.
This is distinct from a (presumably plastic) card, programmed with a fixed sum of money bought by a customer as a gift to a relative, friend or even employee. So a parent of an adult child might buy a John Lewis gift card, to be spent something s/he might want. It takes the form of two cards: the programmed piece of plastic, and the cardboard card with a suitable picture and message and, of course, its envelope.
The distinction is, I think, clear as it stands. But there is ample room for it to become blurred with time and casual usage. After all, both card and voucher stand for a sum of money to be accepted as payment by the issuer. These restrictions, of course, make both different from the credit or debit card.