Incorrect account numbers will be much more of a problem than ABA numbers, as there is no way to validate them and they vary from 5 digits to 16ish. Treat the account number like a password and have the payee enter it twice.

As far as bankdata.io I suggest if you want to keep your site, make it free. Accuity (owned by Fed) is the only entity that is authorized to sell the data (ask me how I know).

The routing number data is published daily @ http://www.fededirectory.frb.org/download.cfm . Be sure to use the "FedACH Participant RDFIs with commercial receipt volume" link. The first link is FedWire, that's not ACH routing numbers.

The list contains previous ABA numbers in case the billed parties bank changes their ABA (this happened to me last year when my bank changed it name). It can be very useful for recurring billing scenarios .

Also, you can catch most ABA issues with a MOD10 LUHN check, rather than a DB lookup.

My last word of advice... Checks can bounce for 30+ days after the transaction. Be prepared to handle this.