PayPal doesn’t seem like a monopoly. Merchants can offer multiple payment processors, and customers can use a different one for each purchase without any lock-in. PayPal has strong competitors like Stripe and even Apple. They also lost their flagship client, eBay.
In some ways they are for consumer products. There is some percentage of people that won’t buy at all without the option. Then a much bigger percent that will still buy but prefer to use PayPal if you give the option.
In fairness, I still wouldn't call that a monopoly, just a popular choice.
I mean, I know some people who won't drink soft drinks unless it's specifically Coca-Cola and know a lot of people who'll pick Coke as their first choice but they clearly don't a monopoly on cola drinks.