Here is the deal. When you use eBay to print a label, one of the options is "shipped from". This normally defaults to the zipcode that is used on your return address. Let's say that your zip code is 53805. If you plan on taking your package to a PO that has a zip code of 53807, you click a button on the eBay shipping label page and change the "shipped from" zip code to 53807. Your return address will remain the same, but the label will show that you mailed the item from a different PO.
The individual PO's are always being pushed to increase their revenue to make their numbers look good. If your shipped from zip code is 53805 then the postage revenue from that package is credited to the 53805 Post Office. If you mail the package from 53807 then 53807 is doing the "work" but they don't receive credit for the postage revenue.
Since the actual work consists of putting the package on the scale and scanning the barcode, it really doesn't amount to much. It really doesn't matter what "shipped from" zipcode you put on the package. The Post Office has to accept the package no matter what zip code is on it.
I wouldn't worry about it. Just nod your head and smile and do what you want to do.
Rick