Adding a yes/no question to shipping page?

Try to make this as least complicated as possible.

I offer UPS and USPS. Haven't had any problems. I would like to make an option box with radio yes and no selections for the shipping page so it can read:

UPS Ground $xxx
USPS priority $xxxx

If you choose UPS Ground and we can ship for less using Priority Mail Flat Rate, would you like us to do so to get some money back? yes/no

The reasoning is that many of the products I sell are heavy BUT they will fit into USPS Flat Rate boxes and will almost always ship for less using them over UPS. Is there a way to add that in or would you not bother? In some cases, this could save customers $5 + dollars in shipping

I suspect that this will prove rather more complicated than it sounds. This thread may provide some help, though what you want to do is a little different,

thanks. Nothing is ever as simple as you want it to be:)