Using stamps.com going from USA to Intl makes it pretty easy. The customs form is automated (you fill in description, price, quant and the HS Tariff number). Recently (several months ago), they switched to everything goes through a global dist center in LA (may be an East Coast one, too) so the label address is for that and there they reship to the international address you put in. I like this a lot because some packages used to get stuck in USPS limbo before leaving the USA (one spent 2 weeks bouncing around the West Coast).
A note on the customs form - getting description and HS Tariff number correct is important. The description can be simple but needs to use words from the actual HS Tariff description. The customs people are not terribly sharp, probably only have seconds to make a decision and just do “simple pattern matching”. For example I sell circuit boards that are used in CNC machines. The HS Tariff section for them is NC Control or some such. So along with the correct HST number, the description is for “NC Control PCB + Assembly Kit”. Never had a problem based on that.
When I diverge from that wording, about 20% get kicked back - usually with “unclaimed” as the problem but I suspect any head scratchers just get tossed into the reject bin. So, keep it simple for the customs people.