Here's the website for U.S. Post Office international flat rate information:
http://www.usps.com/international/pr...ernational.htm
I have not shipped internationally this way. I clicked on "rates" at this website and see that there are weight limitations on each size of box. If you click on "supplies" you will see what the different flat rate containers look like and what the dimensions are.
For flat rate shipping within the U.S., I do not think there are any weight limitations; it's just whatever you can stuff inside the envelope or box. The boxes are free; you can pick them up at the post office. You are not charged the flat rate until you bring the box to the post office to mail.
I think that the problem with shipping fabric from the U.S. to other countries is not so much in the price of the shipping, as that tends to be fairly reasonable, but more in the tax the receiving country charges. To ship internationally, you have to fill out a customs form. If you are shipping fabric to Canada or the U.K., for example, the person receiving the shipment may be required to pay fairly high taxes on it (VAT?). I have shipped a few packages abroad that were gifts, and don't think anyone was charged tax on them because they were labeled as gifts, but you have to be careful about the amount. The only thing I shipped that was worth more than $80 or so were family photos.