The degree to which this is possible has been hugely exaggerated. Yes, you can have DNA mailed to you, but not DNA as its packaged in the human genome.

When you order from NEB or Sigma-Aldritch, what you get is a short DNA sequence (usually on the order of hundreds to thousands of basepairs) in a buffer solution. When forensic scientists do DNA preps of cellular samples from a crime scene or for a paternity test, they purify genomic DNA. This is millions of basepairs coiled around storage proteins to form a densely packed material called chromatin. The procedures used to purify chromosomal DNA would wash away any of the much smaller, synthesized fragments of the sort that you can order from a company, meaning that faking a paternity test in this manner is impossible.

Biotechnology is exciting, but not so exciting as University press offices might have us believe :(

Putting your genome online in this manner is more of a concern for insurance than anything else, I think— although as others have pointed out, discriminatory insurance practices based on preexisting conditions will soon be illegal in the U.S. . . . still, better not to take any chances.