Processing Spinner and Processing notification

I am in need of having a message display after the submit button has been pressed and validation has taken place that states that the transaction is being processed (this is for a donation page). I realize that this could be displayed as a validation message but I am in need of it displaying while the donation is being processed. I have had users think that the site is locking up due to the time taken to process a credit card which resulted in them pressing the submit button again and thus being charged twice. Also the process spinner doesn't seem to appear anymore which I think would be a help in this matter. Does having AJAX animation on affect this? What would be great is to have a modal window pop up that states that the transaction is being processed and may take a moment. If this is not an option then a message that appears above the button would work as well. Any ideas?