I'm looking for services with API's that will do fee free transfers of money between bank accounts, these accounts will both be controlled by the same person, but may not be under the same bank. Each transfer will be small, most less than $5, but some may go higher, there really is no upper bound on the use case. I may be able to batch these transfers for services with fees, if the fee is small enough, but that is less than ideal for my use case.
I know Paypal can do this with a fairly significant delay (10-14 days), but I am looking for all viable options here. I would like to support as many services as possible.
Edit - Additional details
I am a developer exploring the feasibility of creating an application to assist users in adopting good savings habits. To do this I need them to be able to make the decision, hopefully several times a day, to fore-go an impulse buy, and instead transfer that money from their transactional account, to a savings type account.
So I need to support as many user's account set ups as possible, while ideally supporting the least number of API's. Fee's must be as close to 0 as possible as the purpose of the app is to save as much money as possible.