Donate for a Cause is a reputable charity. They donate proceeds to great causes as well. They are the only up front fee company that is trustworthy.
Think of why they are asking you for upfront money, you have a timeshare that you are having trouble even giving away, which led you to consider donating it. What good would this company do for any charity if they took "worthless" timeshares, and then attempted to sell them for a profit so proceeds could benefit a cause. The reason they are asking you for money is to actually secure a donation, as well as to cover any MF incurred while they attempt to give it away for free to some company to sell on ebay.
If getting rid of your timeshare as cheaply as possible is a concern, then explore other avenues as Denise suggested, and hopefully someone will take it off your hands. But if you are still stuck with it after that, you can rest assured at least the money your giving is going to charities such as the american cancer society, cerebral palsy, etc.
also, the tax deduction thing .. its real market value. so unless your timeshare is selling for a couple thousand on ebay, you aren't going to be deducting much if anything.
good luck.