The common example is Mastercard and Visa. They charge a fee to the merchant but they require that that the merchant cannot offer you a lower price to pay some other way to avoid the fee. On top of that, there are 2 fees, one for "regular cards" and one for "premium cards" (gold cards, platinum cards). They charge the merchant more when a user uses a premium card but they require the merchant not to tell you.
In an ideal world (well, my ideal world), these fees would be called out so that you, the customer, could choose not to pay them. You might see, "if I pay cash this $100 product will cost $100 but if pay with my platinum card it's $105. I can save $5 by paying cash".
Mastercard and Visa forbid this for two reasons: (1) they don't want you choose the cheaper cash option (2) they don't want a competitor to be able to offer a lower fee. In the example above, they charge $5 but a competitor could charge $1. You'd might pick the cheaper option but you're unaware of it since they forbid the merchant from offering.
They can only do this because they effectively have a duopoly on the market. Don't play by their rules, lose 99% of your customers. Various governments try to fix this. The cynical reason it hasn't been fixes is they pay off politicians to make sure it doesn't get fixed.