I think flat fees for international shipping (I think AlexN was right suggesting the shipping calculator for the US) are MUCH easier.
A flat fee is only going to cause a problem when it is obvious you are overcharging, but for something as light as your top the difference between the cheapest and most expensive location using Global Priority Mail is only US$3. It is only with heavier items that the differences in price start to be more noticeable (up to US$13).
Still, if you wanted to be completely accurate, there are only five "rate groups" to list: 1 - Canada: US$6; 2 - Mexico US$7; 4 - Australia, Japan and New Zealand US$9; 3 & 5 - Rest of the world US$8 (Group 3: Belgium, Denmark, Finland, France, Germany, UK, Iceland, Ireland, Israel, Liechtenstein, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden & Switzerland. Group 5: Aruba, Austria, Bahamas, Barbados, Brazil, Chile, China, Colombia, Costa Rica, Czech Republic, Dominican Republic, Guyana, Hong Kong, Hungary, India, Jamaica, South Korea, Malaysia, Philippines, Poland, Saint Lucia, Saudi Arabia, Singapore, South Africa, Taiwan, Thailand, Trinidad & Tobago, Turks & Caicos Islands & Vietnam).