AbleCommerce Shopping Cart Software

I want to set up only USPS shipping option for PO Boxes, FPO's and APO's, but I don't want to offer USPS for anyone else. I tried getting zipcodes for all PO Boxes, APO, FPO from the USPS website, but we're having some orders slip through the cracks, meaning that the zipcode list is not comprehensive. Plus, there appears to be some overlap, so UPS Ground is showing up also, which I don't want.

Is there any way I can only allow USPS for these types of addresses without inputting every zipcode in the US? The zones page in the admin times out on me already because we have so many zipcodes in the system, I'm having to manage this with sql queries, which is tedious.

We have just put filtering on the shipping methods in checkout where we check every address for all the possibilities of PO Box we can think of and then don't show UPS methods.

That's what I thought I'd need to do, just wanted to check to see if there was something I was missing that was already packaged. They should build this in in my opinion. Thanks for the answer.