I can't claim to be able to make a sticky with facts and figures, but I can add in my experience of USPS.
Firstly, it can be more difficult for USA members to send internationally than domestic. This is because for domestic mail they can have the postal worker collect it from their home or business, but for the international options they must actually go to a post office. This might not seem like a big deal to Europeans for example, because it's likely that wherever you live, you're never that far from a post office. However, the USA is a BIG place, and many members can live an hour's drive or even further from their nearest post office. An international member offering to pay charges, fees and any shipping is one thing, but asking someone to go 2 or 3 hours drive out of their way on a Saturday is asking too much.
For USA members who are in 'range' of a post office, there are 2 main options, standard first class international and the old faithful, the flat rate priority box/envelope. I wish I'd kept all those envelopes actually, I could probably re-do my roof in glossy USPS colors and still have some left over!
So, first class international. That's put it in a envelope, get the post office to put the right stamps on it, and off it goes. A brief pricing example for EU would be small items, bezel or tailcap etc, around $2.50. Average flashlight (6P) with no packaging or batteries, around $6. Same 6P new in packaging, between $8 and $10. That gives you an example of approximate weights/prices. As you can see, I've been here before!
The flat rate box is $12.95, or $12.30 if paid online, weight up to 4lb (I think) same rate to most countries as far as I know.
Customs forms. Although (as far as I know) this is a relatively little known fact among USA members, standard first class international does not actually require a customs form. However, it seems that many postal staff aren't aware of that and often insist on it anyway. Whether or not it is required on arrival in your country is your business though. In my case I'm quite lucky in that it isn't required at all. I have lost count of how many messages I have sent to sellers with the same old thing "put it in an envelope, stick 2 or 3 bucks of stamps on it, and I'll get it". No doubt some of you reading have received one of these messages from me
USPS flat rate box/envelope does require a customs form.
The best advice I can offer on getting USA members to ship internationally is this, just ask, nicely. I sometimes see people posting angry icons, that sort of thing, when they have put an "I'll take it if....." in a thread clearly marked CONUS only, which is then passed over to a USA buyer. If I were in the USA and put CONUS only and someone ignored that and just put an I'll take it without even asking first, I'd be worried not so much about the fact that they ignored the CONUS only, but what other parts of my thread conditions did they miss, or just choose to ignore?
So, send a message, ask nicely, and if the answer is no, don't push it. No member has to 'justify' why they send where they send, and why they won't send elsewhere.