Lessons 2 and 9 taught the merchant side: someone is paying you, fees cost you money. Issuing is the other half. Your fintech gives users a debit card. When the user swipes it at a merchant, money flows OUT of the user's wallet AND interchange flows IN to you, because every card transaction generates a fee, and a slice of it goes to the issuer. Most engineers only think one direction; the issuing model inverts the mental picture.