Direct Customer to Client Area WITHOUT displaying Invoice when...

Whilst I'm sat here in my back garden, enjoying the 32c weather, I'm starting to break out into a sweat! NOT because it's hot, but because I'm trying to figure out how to direct the customer to the Client Area WITHOUT displaying Invoice depending on which payment method is chosen...

For example:

PayPal
Customer checks out, chooses PayPal - They are taken to the PayPal site to complete payment and are passed back to whatever URL you ask PayPal to pass them back to. (For me, it's https://portal.mydomain.co.uk/clientarea.php) - PERFECT!

Bank Transfer
Customer checks out, chooses Direct Bank Transfer - They are IMMEDIATELY shown the invoice with a text link at the bottom of the invoice page to direct them back to the client area. NOT what I want.

What I WANT is for them to be shown a message (Not bothered how - jQuery popup message perhaps), telling them "You've selected to pay by Direct Bank Transfer - Pay up or you'll be sorry". Then, two nice big buttons - one that says "View Invoice", the other that says "Go to Client Area".

But wait, that's now ALL - When they decide to "View Invoice", I want it to display in a POPUP window and NOT leave the page they are on.

I don't want MUCH, do I..?

Any assistance would earn your choice of cold drink and an hour on my sun lounger :-)