I instituted a chargeback against StubHub, as it changed its customer "guarantee" some time in mid-March and refused to refund for cancelled performances, now offering only a "coupon" valid through the end of 2021. Amex initially reversed the charge, then reinstated it a few months later, pursuant to StubHub's response that the charge was valid because I was provided a coupon pursuant to its March 2020 terms revisions (the purchase was made in fall of 2019 when StubHub guaranteed a cash refund for cancelled performances).
As proof, StubHub submitted an illegible copy of its T&Cs. When I contacted Amex, they acknowledged the copy they received was also illegible, but that there was "nothing they could do" because those were the vendor's stated terms. This certainly seemed like an appropriate situation for a chargeback to me, but StubHub has aggressively responded to chargebacks, BBB complaints, etc. by asserting it can change its terms whenever it wants and they are effective as to the date of the scheduled event and not the date of customer purchase.