Originally Posted by Steve M

A wire payment and an online payment (done through an ACH electronic transfer) are totally different beasts. A wire payment occurs instantly during banking hours, is guaranteed to be good, and basically cannot be undone. An ACH transfer is done in daily batches (like a check), may be easily undone by the sending bank within 2 days if there was a mistake, and for various reasons can be reversed for weeks in the future (the same as a check) if for instance it's determined that it was requested fraudulently by other than the sending account's holder.