*The USPS offers a tool to
calculate shipping, and you can estimate the cost to different destinations for a variety of different services.
*It is important to remember that, for the most part, this First-Class International Package Service may only be used for parcels up to 4 pounds; anything more will have to use the next available service.
*This service would be the one used for bubble mailers (I use boxes for mine for added protection). As far as I understand, anything that has a thickness over 1/4 of an inch is considered a parcel, and so would have to be sent using this service.
*You are allowed up to 36 inches in combined length, width, and height/thickness for a package using this service.
*The weight can slightly alter the shipping cost, so it would be worth it to invest in a scale to have an accurate weight. This is also important for the customs form that must be attached to the parcel, as the weight of the contents must be listed in addition to the total package weight.
*The USPS also has a tool for
generating a customs form to print out and affix to the parcel.
*They also have a page with links to
information and tips on preparing international shipments in general.
*Further, they have
information on individual countries since prohibitions, restrictions, standard practices, and the like may vary by location.
*Tracking is possible when a package is shipped to
certain countries; so far, tracking information has worked extremely well on all packages I've sent to Canada, Belgium, and Spain.
*If you
create an online USPS account, you'll be able to use the Click-N-Ship tool to generate a shipping label and a customs form, as well as obtain the tracking information when applicable.
*I would imagine theoretically that stamps may suffice to cover the shipping cost, but you may sacrifice access to tracking, and it will require a lot of stamps for an international parcel. It's easier to pay online and have the postage accounted for on a printed label.
*The lowest starting price for international shipping that I've seen with this service is $10. For Canada, I have seen anywhere between $10 and $16 (depending upon package weight and size). I haven't shipped to the UK yet, but I believe the starting price is at $14.
*The customs form is extremely important; weight and value especially must be accurate. Citizens of other countries often have to pay fees based on the value of the contents of international parcels that they receive (which is something I didn't know, and I haven't heard of this occurring in the US). This value does not include the shipping cost. So, for example, whatever someone pays you for one of your ponies - let's say $8 - is the value of the parcel. Nothing more, nothing less.
*On that note, for the First-Class Package International Service, I believe that you are not allowed to send anything valued at $400 and over.