The post office in GA does handle parcels ok, though I don't know if there is a size limit or not. A few years ago, I posted a small package back to the UK, it arrived in about a week, can't recall the postage cost now but I don't think it was excessive. Also posted a parcel within Spain, about the same size as a "small parcel" would be in UK, registered, arrived next day.
Best thing would be to call in to the PO and ask, I find the staff there helpful.