As Gully above says, for international customers, the shipping carrier IS important.
If I can, I will always choose USPS first class airmail or priority mail over UPS or FedEx. Both UPS and FedEx are prohibitively expensive, nearly always get searched by Customs, and will only deliver to us if we're home. I work full time and live alone, so I am never home. I recently spent more than a week trying to organize a delivery time for a FedEx/DHL parcel before the thing actually got to my house.
USPS has cost-effective solutions that get most of the things I order from the USA to my PO box within about 10-14 days, no hassles. This is good.