PO Box Ship Methods on eStore

Hi,

We are having an issue where customers are selecting ship methods that don't deliver to PO Boxes in checkout. We have a note that if it is a PO Box you have to select a USPS shipping method, but customers still select UPS on occasion. What we are trying to do is customize reference checkout so that it will only show USPS shipping methods if the address is a PO Box. I am wondering if any of you guys have ran into this issue before? If so how do you handle it?

Our proposed solution is to add a drop down list to the address object that will be populated with something like "PO Box" or "Physical Address". We would then customize reference checkout to show the appropriate ship methods depending on what type of address the customer is using.