I haven't sent any items like this to the US for a little while but previously I was using FedEx via Parcel2go, they seemed to have pretty fair rates and the delivery was always 2-3 days max and tracking is excellent.
You will however need to enclose 3 commercial invoices (parcel2go will guide you through these as you create the order) I never had anyone in the US complain they had to pay Customs fees. However they should be aware it's a possibility and/or check with their local customs and excise dept.
Lastly all Computers, even vintage going to the USA must have 3 copies of an FCC form completed. I'm not sure the A600 has an FCC ID, at least I could never find one but you need to complete the form anyway and tick option 2. Fill out as much of the other info as you can.
You can download the FCC form here:
https://images.fedex.com/gtm/pdf/USF007.pdf
This is only required for electrical devices being sent to USA.