When I modify a payment using the RecievePaymentMod request, I need to supply the txnID of the payment. Then in the response, it sends back a txnID (don't know if it's the same or not, haven't put it into practice yet).
If I have to modify the same payment again, do I use the txnID returned from the original ReceivePaymentAdd request, or the txnID returned from the last RecievePaymentMod request?
don't know if it's the same or not, haven't put it into practice yet
It is.
The TxnID is a unique primary key for QuickBooks, just like your unique primary key in any standard database system.
If I have to modify the same payment again, do I use the txnID returned from the original ReceivePaymentAdd request, or the txnID returned from the last RecievePaymentMod request?
It's the same value. It's a unique key that never changes.