Add some lines for managing the bank's security requirements: signature, encryption and authentication.
If you work with multiple banks, like many corporates or fintechs, multiply many of these lines by the number of banks you work with.
Even before starting to code anything, a big part of the job is obtaining the documentation from each bank and specifying the integration for each bank.
For instance, for the same payment scheme, different banks require different maximum payments per file or payload, or maximum payment file or payload size.