Charities should welcome your gift whenever you want to send it. Question vague appeals. Appeals should clearly identify the charity's programs. Watch out for appeals that are strong on identifying a problem, but weak on describing exactly what the charity plans to do about it. High tech does not always mean high ethics.
Honest charities have easy access to the Internet at low cost, but so do unscrupulous operators.
A fancy web site does not necessarily mean that a charity is above-board. Check out online appeals as you would any other solicitation. Don't hesitate to ask the charity for written information on its programs and finances. This can be particularly helpful in responding to phone appeals from charities that interest you. Do not give cash.