My point is that the regular post office service is probably 10 minutes away from you and it is the same service you get in the US mainland.
If you are at a number/name street and house has a number it is easily located and there should be little need to use any form of PO Box. Even when dealing with "Campo" addresses such as town, road number, kilometer number, the postman tends to learn who lives along his route and can deliver to your house. But "Campo" addresses may be unreliable as it depends on who is doing the route.
Just get a PO Box at the regular post office which is closest to you and is likely to be cheaper than the store. US Federal Laws apply so service should be reasonable.
Good Luck