Sending several payments from the address for payments is not immediately, but in turn. Claims for payment are first placed in a special queue. This is done in order to reduce the probability of receiving errors when sending transactions to the network. Transactions are not instantaneous. They may not go into the network or they can leave, but be ignored by the network. Therefore, until confirmation, you can not be sure that the transaction has been processed.
It is for this reason that we do not notify you of the txid transaction immediately after sending. You will receive it in IPN after confirming the network.