For example, on the Hillary Clinton campaign website, if you click the link to donate and scroll to the bottom of the page, it says this:
By clicking "Donate," I certify that:
* I am a U.S. citizen or lawfully admitted permanent resident of the U.S.
* I am making this contribution on a personal card with my own personal funds, not those of another person or entity.
* I am not a federal contractor.
* I am at least 18 years old.
Contributions or gifts to Hillary for America are not tax deductible.
An individual may contribute up to $2,700 per election (the primary and general are separate elections). By submitting your contribution, you agree that your contribution will be designated for the 2016 general election. By providing your phone number, you are consenting to receive calls, including automated calls to that number from Hillary for America.
Federal law requires us to use our best efforts to collect and report the name, mailing address, occupation and name of employer of individuals whose contributions exceed $200 in an election cycle.
Mail: To contribute by mail, please send a personal check made payable to Hillary for America to:
Hillary for America
P.O. Box 1700
Merrifield, VA 22116-9559
Please include your full name, address, phone number, email address, occupation and employer in the envelope.
If the campaign does not provide such instructions, I think you would be ok if you usedthe instructions from the Clinton campaign as a guide.