Financial transactions are not necessarily "very high privacy" in the U.S. Financial transactions over $10,000 are required to be reported to the government, as well as transactions that a banker believes are part of a cumulative transaction over $10,000.
I don't think it's clear how well credit card transaction data is protected against subpoenas, and there is a rumor that one or more of the many undisclosed §215 bulk collection programs relate to financial transactions. There are statutory protections for financial transaction records, but the Supreme Court has held that a bank account holder has no constitutional expectation of privacy in the bank's records. U.S. v. Miller, 425 U.S. 435 (1976).
To the government is one thing, you have to report all income to the government over a certain threshold. If you are a corporation trying to buy votes, then your finical dealings are highly private.*
https://en.wikipedia.org/wiki/Bank_Secrecy_Act
In addition, banks are not allowed to open accounts without verifying the customer's identity.
https://en.wikipedia.org/wiki/Know_your_customer
I don't think it's clear how well credit card transaction data is protected against subpoenas, and there is a rumor that one or more of the many undisclosed §215 bulk collection programs relate to financial transactions. There are statutory protections for financial transaction records, but the Supreme Court has held that a bank account holder has no constitutional expectation of privacy in the bank's records. U.S. v. Miller, 425 U.S. 435 (1976).