When you say you purchased your ticket online was it at
www.Delta.com?
When you go to any US airline's website you will automatically default to the US website and any purchase you make will be in US Dollars and considered a purchase made in the US. If you are purchasing a ticket and are located in another country you need to switch to the country specific version of the website.
On Delta.com the link is on the top of the page. When you click on "Country/Language" you will receive a drop-down menu which will let you select your country of origin and preferred language. If you do not complete this step you will be purchasing your ticket in US Dollars and all restrictions to a ticket purchased in the US will apply. After you are logged into the country-specific website all fares will be quoted in your local currency and will be considered as purchased in your country of residence.