The Swiss Travel Pass is available at the Tirano station, and I think credit card payment shouldn't be a problem. You can ask the SBB to be sure.
However, it's even easier to buy your Swiss Travel Pass online as an e-ticket and print it at home. I'm not sure when you're leaving, but mind that you have to do this at least 3 days before your departure.
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Swiss Travel Pass promotion 25% discountOnly if purchased in July 2021. Free cancellation.Buy via the SBB travel centre at https://shop.switzerlandtravelcentre.com or HappyRail at https://www.happyrail.com/swiss-travel-pass.