How reliable are DNA paternity tests? We took then at an office, and from there they are being mailed to a lab.

Would that in any way affect the DNA?

There is a following report on MSNBC news.

Pharmacy chain markets DNA paternity tests in 30 states nationwide

Who’s your daddy? DIY paternity test debuts
March 27: A new type of at-home medical test can reveal a child’s paternity.