I straight up hate working in environments which make you use your own “stuff” to do necessary work, be that credit cards, phones, email addresses, laptops etc. Unless you signed up to work on that basis, perhaps as a contractor with your own equipment, it feels like a real imposition. I resented even having to install 2FA on my phone for work and resisted it for some time before giving in to how much easier it made my day.
Fundamentally I feel no employer has the right to financially disadvantage me, to take up space on my devices (or whatever) without my absolute consent, and that realistically they should be paying me for any disadvantage they are causing me - interest on payments taken for example. And I mention all this because I know I’m not alone in feeling this way (others I work with have expressed similar sentiments about previous impositions) and I think it’s a situation which should be handled with some urgency and care.
The last time I worked in an environment where we tested payments, we had a test card to use. I was told this was supplied by the payment vendor. It absolutely worked on live although it did trigger a “fake” authentication somehow, which meant no money was taken and the transaction was somehow flagged in a way we could pick up - sorry for the absence of details, I was very junior at the time! This was ~6 years ago and things may well have changed since then, but it seems strange any payment org wouldn’t make their service testable? Either way, my first action would be to speak with the vendor and find out if they do have a test programme/system of some kind.
If not, next action would be to get the company to issue a card for test transactions. This hasn’t happened already which suggests some hesitation in doing so… presumably you are already providing a lot of hardware, software, office costs etc. I can’t see that a credit card is really much more of a financial burden and it is a necessary tool for doing the job the company needs you(/r testers) to do.