We reference the Cambridge Advanced Learner's Dictionary.
In British English, we write television programme and computer program.
In American English, we write television program and computer program.
NOTE: All examples below gleaned by creatively grepping the OED.
Whenever you create a brand new word by applying a productive prefix to a proper noun, which therefore begins with a capital letter, the hyphen is obligatory.
- miso-Hellene
- pre-Einstein
- pre-Freudian
- pre-Socratic
- post-Cantor
- post-AIDS
- post-Newtonian
- post-Nicene
- post-Pliocene
- anti-American
- ante-Justinianian
- de-Stalinize
- intra-European
- trans-Siberian
- pan-European
- proto-Hittite
- un-American
- pseudo-Gothic
This general rule is especially useful for nonce words, coinages you create on the fly, which might not otherwise be recognized by the reader. When you create a new derived term where each part is normally capitalized, then with time, the hyphen and the second capital tend to become lost.
For example, we oppose the Arctic not with the *ant-Arctic, but with the Antarctic. And while terms like Tibeto-Himalayan or Proto-Indo-European seems resilient to this process, many others are not, including:
- Amerasian
- Amerenglish
- Amerindian
- Eurafrican
- Eurasian
- Mesoamericanist
(There are also a few false positives to be wary of, like Bildungsroman and Künstlerroman, which do not, in fact, involve any Roman elements. :)
Counterexamples, where neither capitals nor hyphens remain, include:
- acatholic — a word the OED equates to non-Catholic
- misogallic
- paganochristian
- rechristianize
- subatlantic
- subarctic
- transatlantic
- uncatholic
- unchristian
- unfrenchify
Some terms enjoy more than one attested spelling. The fully hyphenated and capitalized version tends to occur first historically, as the term is first coined and then popularized, while the smoothed-down version follows once the term has been around a while. So the Pre-Cambrian period of geologic time is now often spelt Precambrian, and the “European climatic period that followed the Arctic and preceded, or marked the transition to, the Boreal period, and was characterized by the spread of birch and pine forests is variously spelt Preboreal, pre-Boreal, and preboreal.
Best Answer
According to this Google Ngram, both E-Commerce and E-commerce are valid and interchangeable:
Likewise, when not beginning a sentence (or in a headline, etc.), the term e-commerce is not capitalized:
See what happens when e-commerce is added to the Ngram.
According to Wikipedia, eCommerce is a viable, though less often used alternative. The Ngram supports that assertion. The only variation which is completely incorrect is ECommerce.
Definition from: New Oxford American Dictionary
Conclusion: